UNIVERSITY  OF  CALIFORNIA 
AT   LOS  ANGELES 


GIFT  OF 

J.    F.    Bovard 


BOOKS 

BY 

JOSEPH  McFARLAND,  M.  D.,  Sc.  D. 


Pathogenic  Bacteria  and  Protozoa 

Octavo  of  8?8  pages,  illustrated. 

Cloth,  $4.75  net.  Ninth  Edition 


Pathology 

Octavo  of  856  pages,  with  437  illustra- 
tions.    Cloth,  $5.00  net.       Second  Edition 


Biology:  General  and  Medical 

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tions. Fourth  Edition 


BIOLOGY 

GENERAL    AND    MEDICAL 


BY 
JOSEPH  McFARLAND,  M.  D.,  Sc.  D. 

•PROFESSOR     OF     PATHOLOGY    AND    BACTERIOLOGY    IN    THE    UNIVERSITY    OF 

PENNSYLVANIA;    FELLOW  OF  THE  COLLEGE  OF  PHYSICIANS 
OF  PHILADELPHIA,  ETC. 


FOURTH  EDITION,  THOROUGHLY  REVISED 


PHILADELPHIA  AND  LONDON 

W.  B.  SAUNDERS  COMPANY 

1920 


Copyright,  1910,  by  W.  B.  Saunders  Company.     Reprinted  July,  1911. 

Revised,  reprinted,  and  recopyrighted  November,  1913.    Revised, 

reprinted,    and    recopyrighted     November,    1916.      Reprinted 

January,  1918,  and    October,   1919.     Revised,  reprinted, 

and  recopyrighted  March,  1920. 


Copyright,  1920,  by  W.  B.  Saunders  Company 


PRESS    OF 

V.     B.     SAUNDERS     COMPANY 
PHILADELPHIA 


TO   MY  MOTHER 
who  first  interested   me  in  the 

LIVING    THINGS 
and  taught  me  to  marvel 
§  at  the 

<J  WORKS    OF    GOD 

U. 


327113 


PREFACE  TO  THE  FOURTH  EDITION. 


"  YES,"  said  my  eminent  friend  to  whom  I  had  con- 
fided the  general  plan  of  this  book,  then  in  the  original 
manuscript,  "  and  who  will  read  such  a  book  as  that?" 

The  question  astonished  me,  but  the  manner  in  which 
it  was  asked  clearly  intimated  that  it  was  a  method  of 
expressing  his  conviction  that  no  one  would  do  so. 

My  friend  is  a  distinguished  educator,  occupies  an  im- 
portant chair  in  one  of  the  most  prominent  American 
institutions,  and  should  have  known;  but  apparently  did 
not.  Time  has  shown  him  to  have  been  mistaken.  Stu- 
dents were  anxious  for  a  writing  that  would  connect  the 
Botany  and  Zoology  they  had  learned  in  college  with  the 
Anatomy  and  Physiology  they  were  learning  in  the  Med- 
ical College,  and  those  subjects  with  the  world  of  living 
things  in  which  they  moved. 

I  still  am  unable  to  answer  the  original  question,  "  who 
will  read  it?"  All  that-  I  can  say,  as  the  result  of  three 
editions  completely  exhausted,  is  that  thousands  are  reading 
it,  and  for  me  that  is  enough  to  justify  the  long  period 
spent  in  its  preparation. 

JOSEPH  McFARLAND. 
PHILADELPHIA,  PA., 
March,  1920. 

11 


PREFACE. 


IN  preparing  this  book  it  has  been  the  purpose  of  the 
author  to  acquaint  his  readers  with  the  peculiar  nature 
and  interesting  reactions  of  "  Living  Substance";  to  help 
him  trace  it  to  its  probable,  though  unknown,  beginnings 
and  follow  it  through  its  multifarious  differentiations 
to  its  highest  complexity. 

In  so  far  as  this  has  been  accomplished,  the  work  is  a 
General  Biology.  But  more  has  been  attempted,  for 
the  problems  have  been  so  considered  as  to  show  that 
man  is  no  separate  entity,  apart  from  the  general  world 
of  living  things,  but  is  a  unit  in  the  general  scheme  of 
things  and  subject  to  the  same  laws  that  apply  through- 
out the  universe. 

Inasmuch  as  many  of  the  subjects  treated  are  of 
importance  to  students  contemplating  future  medical 
studies,  and  inasmuch  as  all  of  them  are  of  interest  and 
importance  to  students  of  medicine  and  physicians,  the 
work  may,  with  justification,  claim  to  be  a  Medical 
Biology. 

All  of  the  problems  of  medical  science  are  in  a  sense 
biological,  and  many  of  the  problems  of  biological  science 
medical.  Medical  science  is,  in  fact,  a  branch  of  biology 
and  should  be  studied  as  such. 

Each  chapter  treats  of  some  subject  or  subjects  upon 
which  the  pen  would  gladly  linger  and  upon  which  a 
volume  might  be  written,  and  professional  biologists 
will,  no  doubt,  be  disappointed  at  the  brief  treatment 
their  pet  theories  receive  as  well  as  astonished  at  the 
space  devoted  to  other,  and  to  them  less  important, 
13 


14  PREFACE. 

matters,  but  this  is  the  inevitable  result  of  the  particular 
point  of  view  of  the  author. 

Nearly  all  of  the  subjects  treated  are  of  controversial 
nature,  but  that  is  the  present  state  of  biological  science. 
Attempts  to  crystallize  incomplete  information  into  laws 
lead  to  theory  rather  than  to  fact,  and  the  subject 
passes  from  theory  to  theory  in  search  of  the  fact. 
This  explains  why  the  consideration  of  certain  subjects 
may  lead  the  reader  to  a  final  interrogation  point  or 
may  end  without  a  personal  expression  by  the  author  in 
favor  of  one  or  the  other  side  of  the  question. 

It  is  hoped  that  the  problems  of  Blood-relationship, 
Infection,  Immunity,  Parasitism,  Inheritance,  Mutila- 
tion, Regeneration,  Grafting,  and  Senescence,  which  have 
been  presented  at  greater  length  than  in  other  writings 
upon  Biology,  may  be  useful  to  the  reader. 

It  is  hoped  that  the  writing  will  not  be  found  too 
technical  to  be  beyond  the  comprehension  of  any  intelli- 
gent reader,  though  it  must  be  admitted  that  some  ac- 
quaintance with  the  sciences  will  be  of  decided  advan- 
tage to  him. 

The  author  expresses  his  sincere  thanks  to  his  friend 
and  colleague,  Professor  Charles  H.  Shaw,  A.M.,  Ph.D., 
for  many  valuable  suggestions  and  criticisms. 

PHILADELPHIA,  PA.  JOSEPH  McFARLAND. 


CONTENTS 


PAGE 

CHAPTER  I. — THE COSMICAL  RELATIONS  OF  LIVING  MATTER.  .  .     17 

CHAPTER  II. — THE  ORIGIN  OF  LIFE 20 

CHAPTER  III. — THE  CRITERIA  OF  LIFE 31 

CHAPTER  IV. — THE  MANIFESTATIONS  OF  LIFE 34 

IRRITABILITY,  34. — CONDUCTIVITY,  67. — MOTION,  71. — 

METABOLISM,  81. — REPRODUCTION,  91 

CHAPTER  V.— THE  CELL 93 

CHAPTER  VI. — CELL  DIVISION 102 

CHAPTER  VII. — THE  HIGHER  ORGANISMS 109 

CHAPTER  VIII. — REPRODUCTION 181 

CHAPTER  IX. — ONTOGENESIS 211 

CHAPTER  X. — CONFORMITY  TO  TYPE 249 

CHAPTER  XL— DIVERGENCE 282 

CHAPTER  XII.— STRUCTURAL  RELATIONSHIP 301 

CHAPTER  XIII. — BLOOD  RELATIONSHIP 317 

CHAPTER  XIV. — PARASITISM 327 

CHAPTER  XV. — INFECTION  AND  IMMUNITY 364 

CHAPTER  XVI.— MUTILATION  AND  REGENERATION 401 

CHAPTER  XVII.— GRAFTING 420 

CHAPTER  XVIII. — SENESCENCE,  DECADENCE,  AND  DEATH 443 

CHAPTER  XIX. — THE  CULTIVATION  OF  TISSUE  IN  VITRO 456 


15 


BIOLOGY:  GENERAL  AND 
MEDICAL. 


CHAPTER  I. 

THE  COSMICAL  RELATIONS  OF  LIVING 
MATTER. 

To  study  the  problems  of  life  apart  from  their  cosmical 
relations  is  to  lose  much  of  their  significance.  It  is  only 
by  an  appreciation  of  the  endless  changes — integrations 
and  disintegrations — that  pervade  the  universe  that  one 
comes  to  realize  that  those  qualities  by  which  we  recog- 
nize living  substance  more  or  less  closely  correspond  to 
the  qualities  of  all  substance,  and  those  forces  by  which 
it  is  animated  to  those  forces  by  which  the  universe  it- 
self is  controlled. 

All  the  demonstrations  of  physics  arrive  at  one  conclu- 
sion: that  the  universe  consists  of  matter  that  is  inde- 
structible, controlled  by  forces  that  are  persistent. 
Beyond  this  it  is  not  in  the  power  of  the  human  intellect 
to  penetrate. 

We  know  nothing  and  probably  never  can  know  any- 
thing of  the  origin  of  matter  or  force,  and  are  obliged 
to  content  ourselves,  as  our  antecedents  have  done,  with 
the  knowledge  that  both  exist,  and  that  we  can  only 
recognize  the  existence  of  force  as  it  influences  matter, 
and  only  know  matter  as  it  is  affected  by  force. 

The  planet  upon  which  we  live  consists  of  matter  in  a 

highly  differentiated  state  which  the  chemists  are  able 

to  resolve  into  a  certain  number  of  forms  so  stable  as  not 

to  be  susceptible  of  further  analysis,  and  therefore  called 

2  17 


18  BIOLOGY:  GENERAL  AND  MEDICAL 

"dements.11  Astronomy,  however,  shows  us  that  the 
primitive  form  of  matter  is  gaseous  and  leads  us  to  infer 
that  it  is  only  through  prolonged  integration  and  differen- 
tiation that  the  "elements"  of  the  chemist  have  been 
produced. 

Of  the  cosmical  theories  the  nebular  hypothesis  of 
Kant  and  Laplace  seems  to  be  the  one  best  suited  to 
the  present  thought,  in  spite  of  the  more  recent  theories 
that  the  heavenly  bodies  including  our  planet  have  been 
formed  by  the  coming  together  of  ice-cold  meteors,  or  by 
gradual  accretion  through  the  continued  accession  of  cold 
planetary  matter  in  space. 

According  to  the  "nebular  hypothesis"  space  is  filled 
with  matter  in  every  conceivable  state  of  integration  and 
disintegration,  its  most  primitive  form  being  that  of 
gaseous  vapor  in  a  state  of  incandescence.  According  to 
fixed  laws,  the  forces  of  the  universe  act  upon  this  gas- 
eous vapor  until  it  gradually  collects  more  and  more 
locally  to  form  nebulous  masses  such  as  can  be  seen  in 
various  of  the  constellations.  Through  infinite  time  the 
nebulae  become  more  and  more  condensed  until,  in  obe- 
dience to  the  continually  operating  forces,  they  begin 
to  rotate  more  and  more  uniformly,  their  vaporous 
particles  to  approximate  more  and  more  closely,  and 
finally  to  coalesce  to  form  fiery  masses  whose  progressive 
integration  passes  from  the  gaseous  to  the  fluid  and 
finally  to  the  solid  state,  and  the  formation  of  definite 
heavenly  bodies. 

When  the  forces  acting  upon  the  nebulous  matter  are 
uniform  a  single  body  may  be  produced,  but  when  they 
are  conflicting  several  may  be  formed,  the  smaller 
rotating  about  the  larger  in  definite  systems.  The 
smaller  bodies  of  the  system  are  subject  to  the  most 
rapid  subsequent  changes,  so  that  in  any  planetary 
system  bodies  in  all  stages  may  be  observed.  This  is, 
for  example,  supposed  to  be  the  state  of  our  own  solar 
system,  in  which  the  sun  is  still  a  gaseo-liquid  incandes- 
cent body,  some  of  the  larger  planets  semi-solid,  the 


THE    COSMICAL   KELATIONS   OF   LIVING    MATTER      19 

earth  solid  and  cool  upon  the  surface,  and  its  moon  prob- 
ably cold  throughout.  It  is  during  the  cooling  and 
integration  of  such  heavenly  bodies  that  the  differen- 
tiation of  the  component  matter  takes  place.  As  this 
progresses,  a  multitude  of  combinations  appear  for  a 
time,  transform  to  new  combinations,  and  so  continue 
through  an  indefinite  series  of  transformations,  eventu- 
ating in  things  constituted  as  we  now  know  them. 

During  these  transformations  certain  substances  appear 
whose  stability  constitutes  the  foundations  of  chemistry. 
Some  of  these  occur  in  the  elementary  form,  i.e.,  incapable 
of  analysis  into  simpler  forms,  but  more  frequently  they 
occur  in  combinations  from  which  they  can  be  liberated 
by  artificial  means  and  thus  reduced  to  the  elementary 
form.  Some  elementary  forms  combine  with  one  another 
easily,  others  with  difficulty.  Some  of  the  combinations 
are  so  unstable  as  to  tend  to  break  apart  rather  than  to 
persist.  Thus,  among  the  chemical  components  of  our 
planet  we  find  a  certain  number,  combined  to  form  the 
rocks  and  soil,  subject  to  little  change,  and  to  that  only 
under  peculiar  circumstances,  while  upon  the  surface  of 
the  earth  we  find  a  small  quantity  of  matter  composed 
of  elements  entering  into  loose  combinations  and  tend- 
ing to  perpetual  change.  The  substances  we  call  living 
are  included  among  these  ever-changing  combinations. 

The  most  evanescent  of  these  compounds  comprise  a 
group  known  as  "  colloids,"  many  of  which  have  a  molec- 
ular composition,  ascending  in  complexity  until  among 
the  proteins  we  find  "protoplasm"  with  a  composition 
not  yet  definitely  determined,  but  embracing  O,  H,  C,  N, 
S,  and  in  some  cases  P. 

This  substance,  protoplasm,  constitutes  the  basis  of 
living  matter. 


CHAPTER  II. 
THE  ORIGIN  OF  LIFE. 

The  early  philosophers  of  all  nations  referred  human 
existence  directly  to  our  parents  and  indirectly  to  the 
gods,  although  it  seemed  to  the-m  a  simple  matter  that 
the  lower  forms  of  life  should  come  into  being  de  novo. 
Belief  in  the  spontaneous  generation  of  the  lower  forms 
of  life  accentuated  as  philosophy  slowly  separated  itself 
from  religion.  Greek  philosophy  is  replete  with  expres- 
sions regarding  the  spontaneous  generation  of  the  lower 
forms  of  life,  and  the  idea  persisted  through  the  middle 
ages  to  become  a  matter  of  paramount  interest  during 
the  latter  half  of  the  nineteenth  century. 

The  most  superficial  consideration  of  the  subject  is 
sufficient  to  show  that  among  the  ancients  it  was  un- 
familiarity  with  the  lowly  forms  of  life  that  led  to  such  a 
notion,  but  more  modern  writers  seem  to  have  been 
led  astray  through  the  expansion  of  knowledge  following 
the  invention  of  the  microscope  which  introduced  them 
to  a  world  of  newer  and  simpler  forms  of  life,  and  thus 
changed  the  problem.  Thus,  one  possessed  of  the  most 
elementary  information  upon  natural  history  might 
scout  the  idea  that  such  complexly  organized  beings 
as  mice  could  be  spontaneously  generated,  though  the 
same  difficulties  might  not  at  first  stand  in  the  way  of 
conceiving  that  amoeba  or  bacteria  might  be. 

As  the  conception  of  spontaneous  generation  has 
undergone  modifications  consistent  with  the  evolution 
of  knowledge  in  general,  it  is  worth  while  to  review  the 
subject  and  see  what  it  has  meant,  and  what  it  now 
means  to  those  who,  in  spite  of  all  the  evidence  at  hand, 
continue  to  adhere  to  it. 

20 


THE   ORIGIN   OF   LIFE  21 

Among  the  ancient  Greeks,  Anaximander  believed  that 
animals  arose  through  the  stimulating  action  of  mois- 
ture. Empedocles  believed  that  all  living  things  arose 
spontaneously.  Aristotle,  whose  familiarity  with  nat- 
ural history  was  much  broader,  does  not  subscribe  to  so 
general  a  view,  but  asserts  that  ''sometimes  animals  are 
formed  in  putrefying  soil,  sometimes  in  plants,  and  some- 
times in  the  fluids  of  other  animals." 

Virgil  in  Book  IV  of  the  "Georgics"  describes  the 
spontaneous  generation  of  bees  in  the  following  language: 

"First,  a  space  of  ground  of  small  dimensions,  and  narrowed 
for  this  purpose  is  chosen;  this  they  cover  in  with  the  tiling  of  a 
narrow  roof  and  with  confining  walls,  and  add  four  openings  with  a 
slanting  light  turned  toward  the  four  points  of  the  compass. 
Then  a  bullock,  just  arching  his  horns  upon  his  forehead  of  two 
years  old,  is  sought  out;  whilst  he  struggles  fiercely,  they  close  up 
both  his  nostrils  and  his  mouth;  and  when  they  have  beaten  him 
to  death,  his  battered  carcass  is  macerated  within  the  hide  which 
remains  unbroken.  Then  they  leave  him  in  the  pent-up  chamber, 
and  lay  under  his  sides  fragments  of  boughs,  thyme,  and  fresh 
cassia.  This  is  done  when  first  the  zephyrs  stir  the  waves,  before 
the  meadows  blush  with  new  colors,  before  the  twittering  swallow 
suspends  her  nest  upon  the  rafters.  Meanwhile,  the  animal  juices, 
warmed  in  the  softened  bones,  ferment:  and  living  things  of 
wonderful  aspect,  first  devoid  of  feet,  and  in  a  little  while  buzzing 
with  wings,  swarm  together,  and  more  and  more  take  to  the  thin 
air,  till  they  burst  away  like  a  shower  poured  down  from  summer 
clouds;  or  like  an  arrow  from  the  impelling  string,  when  the  swift 
Parthians  first  begin  to  fight." 

Ovid,  in  his  poetic  account  of  the  Pythagorean  philos- 
ophy, commits  its  followers  to  belief  in  many  forms  of 
spontaneous  generation: 

"  By  this  sure  experiment  we  know 
That  living  creatures  from  corruption  grow: 
Hide  in  a  hollow  pit  a  slaughtered  steer, 
Bees  from  his  putrid  bowels  will  appear, 
Who  like  their  parents,  haunt  the  fields  and 
Bring  their  honey-harvest  home,  and  hope  another  spring. 
The  warlike  steed  is  multiplied  we  find, 


22  BIOLOGY:   GENERAL   AND   MEDICAL 

To  wasps  and  hornets  of  the  warrior  kind, 
Cut  from  a  crab  his  crooked  claws  and  hide 
The  rest  in  earth,  a  scorpion  thence  will  glide, 
And  shoot  his  sting;  his  tail  in  circles  toss't 
Refers  the  limbs  his  backward  father  lost; 
And  worms  that  stretch  on  leaves  their  filmy  loom 
Crawl  from  their  bags  and  butterflies  become. 
The  slime  begets  the  frog's  loquacious  race; 
Short  of  their  feet  at  first,  in  little  space 
With  arms  and  legs  endued,  long  leaps  they  take, 
Raised  on  their  hinder  parts  and  swim  the  lake, 
And  waves  repel;  for  nature  gives  their  kind, 
To  that  intent,  a  length  of  legs  behind." 

During  the  middle  ages  Cardan,  in  1524,  declared  that 
water  engendered  fishes,  and  that  many  animals  spring 
from  fermentation.  Van  Helmont  published  special 
directions  for  the  experimental  generation  of  mice. 
Kircher  describes  and  figures  certain  animals  which  he 
declares  were  formed,  under  his  own  eyes,  through  the 
transforming  influence  of  water  upon  the  stems  of 
plants. 

Children  and  ignorant  persons  still  believe  in  the 
spontaneous  generation  of  many  living  things  and  in  the 
country  districts  many  persons  of  otherwise  good 
judgment  believe  that  frogs  and  mosquitoes  arise  spon- 
taneously in  marshes,  while  the  belief  that  a  horse-hair 
placed  in  a  water  trough  will  be  transformed  to  a  thread- 
like worm,  is  widespread. 

No  one  seems  to  have  doubted  that  maggots  were 
spontaneously  developed  in  putrid  meat  until  Redi 
became  interested  in  the  subject  about  1680  and  dis- 
proved it  by  a  simple  scientific  demonstration: 

"  Watching  meat  in  its  passage  from  freshness  to  decay,  prior 
to  the  appearance  of  maggots,  he  invariably  observed  flies  buzzing 
around  the  meat  and  frequently  alighting  upon  it.  The  maggots, 
he  thought,  might  be  the  half-developed  progeny  of  these  flies. 
Placing  fresh  meat  in  a  jar  covered  with  paper,  he  found  that 
although  the  meat  putrefied  in  the  ordinary  way  it  never  bred 
maggots,  while  meat  in  open  jars  soon  swarmed  with  them.  For 


THE   ORIGIN   OF   LIFE  23 

the  paper  he  substituted  fine  gauze  through  which  the  odor  of  the 
meat  could  rise.  Over  it  the  flies  buzzed,  and  upon  it  they  laid 
their  eggs,  but  the  meshes  being  too  small  to  permit  the  eggs  to 
fall  through,  no  maggots  generated  in  the  meat,  they  were,  on 
the  contrary,  hatched  on  the  gauze.  By  such  a  series  of  experi- 
ments Redi  destroyed  the  belief  in  the  spontaneous  generation  of 
maggots  in  meat  and  with  it  many  related  beliefs." 

But  in  1683  a  new  phase  of  the  subject  was  opened  by 
the  discovery  of  bacteria  and  many  other  minute  forms 
of  life  by  Leeuwenhoek,  and  many  scientific  men  to 
whom  the  spontaneous  generation  of  fishes,  frogs,  or 
insects  appeared  to  be  an  absurdity,  were  misled  into 
believing  that  what  was  not  possible  for  these  highly 
organized  animals  might  easily  take  place  among  such 
minute  and  lowly  organized  beings  as  those  of  the  new 
world  disclosed  by  the  microscope.  It  seemed  as  if  the 
threshold  of  life  had  been  reached. 

To  the  intelligent  mind  of  the  present  day,  disem- 
barrassed of  erroneous  beliefs,  it  will  at  once  appear  that 
the  size  of  the  creatures  has  no  influence  upon  the  merits 
of  the  case,  for  there  are  minute  organisrns  visible  only 
to  the  microscope  that  are  as  complexly  formed  as  others 
that  may  be  inches  or  even  feet  in  length.  To  the  idea 
of  simplicity  of  structure  the  mind  may  yield;  it  does  at 
first  seem  more  reasonable  that  an  organism  of  extreme 
simplicity  might  arise  spontaneously  than  that  one  of 
great  complexity  could  do  so. 

So  it  seemed  to  these  early  scientists.  The  micro- 
scopic organisms  were  small,  and  in  many  cases  of  no 
visible  complexity,  and  with  few  exceptions  they  seemed 
satisfied  to  believe  that  they  arose  de  novo.  So,  in  the 
nineteenth  century  we  find  the  same  convictions  regarding 
the  spontaneous  generation  of  the  slightly  known  forms 
of  microscopic  life  that  had  been  held  hundreds  of  years 
before  regarding  the  slightly  familiar  small  but  complex 
animals,  and  thousands  of  years  before  regarding  all 
living  beings. 

But  as  time  went  on  the  world  of  microscopic  life  was 


24  BIOLOGY:  GENERAL  AND  MEDICAL 

scanned  with  improved  instruments,  and  its  population, 
when  studied  and  classified,  proved  to  be  a  miscellane- 
ous one  with  varying  structural  complexity  descending 
to  a  group  so  simple  that  correct  classification  seemed 
impossible.  These  ultimate  beings  appeared  to  be 
neither  animals  nor  plants,  and  to  have  no  definite  place 
in  the  general  scheme  of  living  things.  They  were  also 
innumerable,  and  their  distribution  apparently  universal. 
Small  wonder  that  it  should  have  been  thought  a  simple 
matter  that  what  had  so  little  structure,  and  was  neither 
animal  nor  vegetable,  could  arise  spontaneously  under 
appropriate  conditions.  And,  in  passing,  let  it  be 
noted  that  the  appropriate  conditions  under  which  they 
appeared  thus  to  arise  were  to  be  found  in  fermentation, 
putrefaction,  and  the  discharges  from  morbid  tissues — 
conditions  that  must  recall  once  more  the  former  beliefs 
about  maggots,  etc. 

But  there  were  some  who  conceived  that  the  relation- 
ship between  these  minute  entities  and  the  conditions 
under  which  they  were  found  were  the  reverse  of  those  so 
generally  accepted,  and  that  instead  of  the  minute  organ- 
isms being  generated  through  fermentative,  putrefactive, 
and  morbid  conditions,  the  organisms  initiated  those 
conditions,  increased  in  number  as  they  progressed,  and 
died  out  as  they  ceased. 

Plenciz,  a  Viennese  physician,  greatly  interested  in 
the  discoveries  of  Leeuwenhoek,  was  one  of  the  first  who 
assumed  a  causal  relation  between  microorganisms  and 
infectious  diseases,  and  published  this  view  as  early  as 
1762.  He  also  believed  that  decomposition  only  took 
place  when  the  decomposable  material  became  coated 
with  a  layer  of  organisms,  and  could  only  proceed  as 
they  increased  and  multiplied. 

Needham,  in  1749,  firmly  held  to  the  belief  that 
animalculae  generated  spontaneously  as  a  result  of  vegeta- 
tive changes  in  the  substances  in  which  they  were  found. 
He  maintained  that  the  bacteria  that  were  seen  to  appear 
around  a  grain  of  barley  kept  in  a  carefully  covered 


THE    ORIGIN    OF   LIFE  ,  25 

watch-glass,  developed  through  changes  incidental  to  its 
germination  in  the  barley  grain  itself. 

Spallanzani,  some  years  later  (1777),  pointed  out  the 
slip-shod  methods  under  which  most  of  the  so-called 
experiments  had  been  performed,  and  supposed  that  he 
had  proved  spontaneous  generation  impossible  by  a  new 
and  improved  technic.  He  filled  flasks  with  various 
organic  infusions,  such  as  were  supposed  to  be  "  biogenic," 
subjected  the  contents  to  thorough  boiling,  hermetically 
sealed  them,  and  then  placed  them  under  what  were 
supposed  to  be  conditions  favorable  to  the  development 
of  life,  but  always  with  negative  results.  Instead  of 
carrying  conviction  with  them,  these  experiments  of 
Spallanzani  were  severely  criticized  by  Treviranus  on  the 
ground  that  the  atmosphere  so  essential  to  life  had  been 
excluded  from  the  fluids.  To  overcome  this  objection, 
Spallanzani  gently  tapped  his  flasks  so  as  to  produce 
minute  cracks  through  which  air  might  enter.  When 
this  was  done,  life  invariably  appeared  and  decomposi- 
tion occurred. 

The  problem  remained  in  about  the  same  state  until 
Schultze  in  1836  improved  the  method  by  which  air  was 
admitted  to  the  flasks.  He  filled  them  but  half  full  of 
putrescible  fluids,  boiled  them  thoroughly  to  destroy 
such  life  as  they  might  already  contain,  and  then  daily 
sucked  into  them  a  certain  quantity  of  air  that  passed 
through  a  series  of  bulbs  containing  concentrated  sul- 
phuric acid  or  strong  alkalies  by  which  any  germs  of  life 
that  might  be  in  the  air  should  be  destroyed.  The  cul- 
ture flasks  were  kept  from  May  to  August,  air  being 
passed  into  them  daily,  yet  without  the  appearance  of 
life  or  putrefaction  in  the  contained  fluid. 

Schwann  in  the  following  year  (1837)  performed  a 
similar  experiment  with  the  same  result,  passing  the  air 
admitted  to  the  flasks  through  highly  heated  tubes 
instead  of  through  acids  and  alkalies. 

Schroeder  and  van  Dusch,  in  1854,  discovered  that  if 
the  mouth  of  the  flask  containing  a  putrescible  fluid  was 


26  BIOLOGY:  GENERAL  AND  MEDICAL 

protected  by  a  plug  of  cotton-wool  through  which  an 
abundance  of  air  could  freely  enter  and  exit,  but  by 
which  it  would  be  filtered,  no  life  appeared  in  the  contents. 
The  investigation  was  continued  in  1861  by  Pasteur, 
who  showed  that  if  the  neck  of  a  flask  containing  putres- 
cible  fluid  was  drawn  out  into  a  fine  tube,  bent  down  along 
the  side  of  the  flask,  and  then  bent  up  again  so  as  to  form 
a  V,  it  could  be  left  open,  after  the  contents  of  the  flask 
had  been  thoroughly  boiled,  without  danger  of  contami- 
nation from  the  outside  air,  which,  entering  through  the 


FIG.  1. — Flask  used  by  Pasteur  in  his  experiments  upon  the  spontaneous 
generation  of  life.  It  was  filled  through  the  top  which  was  then  sealed.  The 
contents,  which  consisted  of  putrescible  fluid  were  then  boiled,  the  side  neck 
being  open  to  permit  the  air  to  enter  and  exit.  As  the  fluid  cooled  the  side 
neck  became  closed  by  a  few  drops  of  water  of  condensation  and  prevented  any 
germs  of  life  from  entering  from  without. 

tubulature,  would  have  any  germs  it  might  contain 
arrested  by  the  drop  of  water  of  condensation  that  always 
collected  in  the  angle  of  the  tube. 

Tyndall  performed  a  most  interesting  series  of  experi- 
ments in  which  tubes  were  so  placed  as  to  project  below 
the  bottom  of  a  closed  chamber  having  a  glass  front  and 
a  glass  window  in  each  side.  A  rubber  diaphragm  was 
fixed  in  the  roof  -through  which  a  tube  passed.  Tyndall 
found  that  a  ray  of  light  passed  through  the  side  windows 
of  the  chamber  was  visible  from  the  front  because  it  was 
reflected  from  the  dust  particles  suspended  in  the  atmos- 


THE   ORIGIN   OP  LIFE 


27 


phere  of  the  box.  After  permitting  the  closed  box  to 
stand  for  a  sufficient  time,  this  dust  was  found  to  settle 
and  the  ray  of  light  being  passed  through  the  side  widow, 
finding  nothing  to  reflect  it,  was  no  longer  visible.  When 
the  contained  atmosphere  attained  to  this  optical  test  of 


m 


FIG.  2. — Tyndall's  chamber  for  investigating  the  spontaneous  generation 
of  life.  The  front  is  of  glass,  as  are  the  side  windows,  w,  w.  The  optical  test  for 
the  purity  of  the  contained  atmosphere  is  made  by  passing  a  powerful  beam  of 
light  from  the  lamp,  /,  through  the  side  windows.  When  the  atmosphere  con- 
tains no  suspended  particles,  the  tubes  in  the  bottom  are  filled  through  the 
pipette,  pc.  (Tyndall.) 

purity,  the  tubes  fixed  in  the  bottom  were  cautiously 
filled  with  putrescible  fluids  through  the  tube  in 
the  rubber  diaphragm.  When  filled,  these  tubes,  the 
bottoms  of  which  it  will  be  remembered  projected  below 
the  bottom  of  the  chamber,  were  heated  by  applying  a 
pan  filled  with  hot  brine,  and  their  contents  boiled  briskly 


28  BIOLOGY:  GENERAL  AND  MEDICAL 

for  a  time.  When  the  chamber  thus  prepared  was  stood 
away,  it  was  found  that  life  rarely  developed  in  the  con- 
tents of  the  tubes  and  that  no  putrefaction  took  place. 
Thus  Tyndall  confirmed  the  work  of  Pasteur  who  in  the 
meantime  had  been  busily  engaged  in  showing  that  there 
were  ''organized  corpuscles"  in  the  atmosphere — the 
"floating  matter"  of  Tyndall — which  when  admitted  to 
the  infusions  caused  them  to  putrefy. 

Cohn  had  engaged  in  morphological  studies  of  the 
bacteria  and  other  low  forms  of  life,  and  had  discovered 
that  many  of  them,  under  appropriate  conditions,  pass 
into  a  resting  or  spore  stage.  In  many  of  the  rod-shaped 
organisms  a  spot  appeared  in  the  rod,  grew  larger  and 
larger,  and  became  surrounded  by  a  capsule.  While 
this  was  perfecting  its  development,  the  rod  in  which  it 
formed  began  to  degenerate  and  eventually  set  it  free. 
Thus  there  came  into  being  a  minute  body — a  spore. 
Further  study  of  the  spores  showed  that  they  abounded 
in  the  atmosphere  and  that  many  of  them  could  endure 
temperatures  higher  than  that  of  boiling  water.  It  now 
seems  clear  that  it  was  through  the  entrance  of  such 
spores  into  the  infusions,  their  endurance  of  the  tempera- 
tures to  which  the  fluids  were  subjected  during  boiling, 
and  their  subsequent  germination  that  the  appearance 
of  life  in  the  fluids  was  to  be  referred. 

Thus  Harvey's  law  omne  vivum  ex  ovo  which  had  long 
been  accepted  with  reference  to  the  higher  beings  became 
applicable  to  the  lowest  organisms  in  the  modified  form 
omne  vivum  ex  vivo,  and  the  doctrine  of  the  spontaneous 
generation  of  life  might  be  supposed  to  have  received  its 
death  blow.  The  evidences  thus  collected  were  subse- 
quently investigated  by  a  great  number  of  workers,  by 
a  great  variety  of  methods,  but  with  uniform  results 
and  at  present  almost  every  scientific  mind  is  satisfied 
that  life  in  the  forms  in  which  we  now  know  it  is  never 
spontaneously  evolved. 

However,  it  remained  to  explain  how,  if  all  life  de- 
scended from  antecedent  life,  living  things  originally 


THE    ORIGIN    OF   LIFE  29 

made  their  appearance  upon  the  earth,  and  to  those 
working  under  the  influence  of  this  necessity  the  new 
doctrine  omne  vivum  ex  vivo  was  not  adequate. 

The  primordial  appearance  of  life  is  too  important 
a  matter  to  be  entirely  neglected,  so  we  are  led  to  inquire 
whether  the  simplest  forms  of  life  known  to  us  are  in  any 
sense  primitive  or  whether  they  may  not,  in  fact,  be 
highly  developed  compared  to  the  primordial  forms  from 
which  they  may  have  descended. 

Here  we  are  confronted  by  several  difficulties,  one  of 
the  chief  of  which  is  that  our  conception  of  "life"  and  of 
"living  substance"  is  based  upon  those  forms  with 
which  we  are  familiar,  and  whose  manifestations  we  are 
accustomed  to  describe  as  vital.  It  does  not  by  any 
means  follow  that  only  such  are  "  alive,"  but  it  is  only  of 
such  that  we  speak  as  alive,  and  only  such  that  we  can 
through  the  limitations  of  our  conception  of  the  term 
prove  to  be  so. 

It  is  also  of  some  interest  to  inquire  whether  the  phe- 
nomena of  life  are  so  different  from  other  chemical  and 
physical  phenomena  as  to  make  us  place  the  customary 
gulf  between  the  living  and  not  living,  or  whether  they 
are  not  but  a  part  of  those  universal  phenomena  by 
which  we  can  in  a  certain  sense  attribute  life  to  the  world, 
to  the  solar  system  or  to  the  universe  itself! 

It  is  almost  certain  that  life  is  no  longer  being  gener- 
ated, and  that  its  original  appearance  upon  this  planet 
took  place  under  circumstances  no  longer  existing. 
Thus,  conditions  of  temperature  during  past  periods  of 
the  world's  evolution  are  believed  by  many  to  have 
been  responsible  for  molecular  combinations  impossible 
at  the  present  time.  This  is,  however,  conjectural,  and 
not  demonstrable.  The  chief  argument  in  its  favor 
is  that  all  of  our  endeavors  to  see  protoplasm  come 
into  being  independently  of  antecedent  life,  have  failed. 
We  are  thus  obliged  to  conclude  either  that  life  never 
.did  arise  spontaneously,  or  that  it  can  no  longer  be  gen- 
erated spontaneously,  or  that  we  are  at  present  unable 


30  BIOLOGY:  GENERAL  AND  MEDICAL 

to  recognize  the  most  primitive  forms  in  which  it  occurs, 
being  acquainted  only  with  what  are  by  comparison 
highly  evolved  forms. 

REFERENCES. 

JOHN  TYNDALL:     "Floating  Matter  in  the  Air,"  N.  Y.,  1882. 

J.  BUTLER  BURKE:     "The  Origin  of  Life,"  N.  Y.,  1906. 

H.  CARLTON  BASTIAN:  "The  Modes  of  Origin  of  Lowest  Organ- 
isms, etc.,"  Lond.,  1871.  "The  Nature  and  Origin  of 
Living  Matter,"  Lond.,  1905. 


CHAPTER  III. 
THE  CRITERIA  OF  LIFE. 

Laying  aside,  for  the  present,  all  speculation,  as  to  the 
connecting  links  between  the  matter  that  we  call  living, 
and  that  which  we  declare  to  be  not  living,  it  becomes 
necessary  to  establish  certain  criteria  by  which  the 
former  be  recognized.  These  distinctive  properties  have 
been  formulated  by  Huxley  as  follows : 

1.  Its  chemical  composition. 

The  chemical  composition  of  living  substance  is  based 
upon  a  complex  combination  of  O,  H,  N,  and  C  known 
as  protoplasm.  It  is  a  protein  that  is  entirely  unknown 
except  as  a  product  of  living  substance.  Its  exact  com- 
position is  not  determined  because  it  is  scarcely  possible 
to  study  it  apart  from  other  elements  by  which  and 
through  which  many  of  its  functions  are  carried  on. 
Chief  among  these  are  S  and  P. 

2.  7/s  universal  disintegration  and  waste  by  oxidation; 
and  its  concomitant  reintegration  by  the  intussusception 
of  new  matter. 

Life  is  accompanied  by  the  manifestation  of  energy 
which  implies  combustion  by  oxidation,  chemical 
disintegration  of  the  complexly  organized  protoplasm, 
and  its  reduction  into  more  highly  oxidized  but  simple 
compounds,  such  as  carbonic  oxide  and  water.  This 
would  soon  result  in  complete  destruction  of  the  proto- 
plasm by  analysis  were  it  not  within  the  power  of  the 
living  substance  to  make  good  this  loss  as  rapidly  as  it 
occurs. 

When  the  living  substance  is  young,  the  function  of 
synthesis  takes  precedence  over  analysis  and  the  organ- 
ism is  said  to  grow.  This  growth  is,  however,  entirely 
31 


32  BIOLOGY:  GENERAL  AND  MEDICAL 

dissimilar  to  that  of  the  growth  of  crystals,  for  example, 
where  it  takes  place  by  accretion,  or  the  addition  of 
new  matter  to  the  surface,  for  it  pervades  the  entire 
molecular  structure  of  the  protoplasm  by  the  actual 
interposition  of  the  new  molecules  between  those  already 
existing.  When  the  function  of  synthesis  keeps  pace 
with  that  of  analysis,  growth  ceases,  and  when  analysis 
is  more  rapid  than  synthesis,  life  soon  becomes  extinct 
and  the  protoplasm  breaks  up  into  simpler  and  simpler 
compounds. 

3.  Its  tendency  to  undergo  cyclical  changes. 

The  cyclical  changes  are  largely  incidental  to  the 
phenomena  of  reintegration.  Coming  into  being  through 
the  activity  of  antecedent  living  substance,  the  living 
organism  proceeds  to  increase  its  own  substance,  to 
dispose  of  that  which  is  formed  in  excess  of  its  own 
needs  by  detaching  it  in  the  form  of  new  individuals, 
and  finally  when  no  longer  able  to  maintain  the  equi- 
librium of  reintegration  to  disintegration,  ceasing  to  live 
and  undergoing  dissolution  by  destructive  analysis, 
but  being  survived  by  descendants  behaving  in  the  same 
manner. 

The  successful  application  of  these  criteria  for  the 
recognition  of  life  presuppose  some  acquaintance  with 
the  supposed  living  substance.  One  cannot  immediately 
employ  them  for  the  determination  of  the  living  or  not 
living  character  of  any  particular  object  picked  up 
haphazard  during  a  ramble  through  the  country,  for, 
should  the  objects  in  question  be  inactive  forms  of  life, 
such  as  the  seeds  of  plants,  not  only  would  confusion 
arise  from  the  evident  dissimilarity  of  chemical  composi- 
tion occasioned  by  the  presence  of  the  starch,  cellulose, 
and  wooden  materials  forming  the  most  conspicuous 
structures  of  the  seed,  but  one  would  be  at  a  total  loss 
in  an  attempt  to  immediately  accord  to  the  seed  any 
molecular  activity  or  cyclical  changes.  A  superficial 
acquaint  ance  with  seeds  is,  however,  sufficient  to  enable 
the  investigator  to  apply  the  second  and  third  criteria 


THE    CRITEKIA   OF   LIFE  33 

without  difficulty,  for  if  warmth  and  moisture  be  sup- 
plied the  living  seed  begins  to  germinate,  a  plant  de- 
velops, and  in  the  course  of  time  new  seeds  identical  with 
that  under  observation  are  formed.  This  shows  that 
life  presents  various  phases  of  activity  and  passivity, 
both  of  which  must  be  taken  into  account  in  biological 
study.  Unfamiliarity  with  the  passive  forms  of  minute 
organisms  was  one  of  the  most  potent  factors  by  which 
belief  in  their  spontaneous  generation  was  kept  alive. 

Life  is  most  evident,  and  hence  best  known  in  its 
active  state.  The  passive  state  not  infrequently  escapes 
or  eludes  observation  so  that  a  much  broader  acquaint- 
ance with  living  things  is  necessary  to  appreciate  it  and 
understand  its  significance. 

Passivity  is  observed  among  both  plants  and  animals, 
though  among  the  latter  it  is  confined  to  comparatively 
few  and  usually  to  the  lowest  forms.  It  may  be  regarded 
as  a  state  in  which  the  living  organism  becomes  capable 
of  withstanding  conditions  incompatible  with  active 
existence.  Thus,  the  cold  of  winter,  the  dryness  of  the 
desert,,  and  the  failure  of  the  food  supply  are  probable 
factors  influencing  the  passage  of  living  organisms  from 
the  active  to  the  passive  state,  and  the  warmth  of  sum- 
mer, the  coming  of  rain,  and  the  presence  of  food,  factors 
in  bringing  them  once  more  to  a  state  of  activity. 

REFERENCE. 

THOMAS   H.    HUXLEY:     "Anatomy  of  Invertebrated    Animals," 
N.  Y.,  1885. 


CHAPTER  IV. 

THE  MANIFESTATIONS  OF  LIFE. 
IRRITABILITY. 

Irritability  is  that  property  of  living  substance  by 
virtue  of  which  it  responds  to  stimulation.  It  is  a 
universal  and  fundamental  characteristic,  and  forms  the 
starting-point  of  all  vital  manifestation.  The  behavior 
of  living  substance  is  determined  by  the  stimulations 
it  receives,  and  without  stimulation  it  does  nothing  and 
cannot  be  recognized  as  living.  When  matter  is  no 
longer  irritable,  and  when  it  fails  to  respond  to  stimula- 
tion, we  declare  it  to  be  dead,  or  no  longer  living. 

The  stimuli  that  excite  the  irritable  reactions,  and  thus 
govern  and  determine  the  vital  manifestations  are  of 
two  kinds:  1.  Intrinsic,  inherited,  and  regulating,  and  2. 
extrinsic  and  modifying. 

Stimuli  of  the  first  class  determine  that  the  living 
thing,  regardless  of  its  simplicity  or  complexity,  shall 
conform  to  a  certain  type,  perform  certain  functions,  pass 
through  a  definite  cycle  of  existence,  and  impart  a  cer- 
tain amount  of  its  substance  to  new  individuals  of  its 
own  kind  by  whom  it  may  be  survived.  Those  of  the 
second  class  initiate,  accelerate,  retard,  or  modify  these 
effects. 

Every  living  thing  is  thus  the  creature  of  circumstance, 
dominated  and  controlled  by  inheritance  and  environ- 
ment. 

The  action  of  stimuli  may  be  continuous,  intermittent, 
or  occasional,  as  to  time;  normal,  deficient,  or  excessive 
as  to  intensity,  and  beneficial  or  injurious  according  to 
duration,  intensity,  and  quality. 
34 


THE    MANIFESTATIONS    OF   LIFE 


35 


Irritability  was  first  recognized  and  is  most  easily 
demonstrated  and  best  known  in  its  most  exaggerated 
forms,  where  the  response  to  the  stimulation  appears 
to  be  disproportionate  to  its  intensity. 

This  has  led  to  the  erroneous  impression  that  mani- 


FIG.  3. — Venus'  Fly-trap  (Dioncea  muscipuLa).  An  insectivorous  plant. 
The  traps  for  catching  the  insects  are  at  the  tips  of  the  leaves  and  consist  of 
two  valves  with  spinous  edges.  At  the  centre  of  each  valve  are  several  small 
spines  which  act  as  triggers.  When  an  insect  disturbs  several  of  these  the  trap 
springs  and  it  is  caught  and  compressed  between  the  valves.  An  enzymic  se- 
cretion is  soon  poured  out  by  glands  in  the  valves  and  the  insect  is  slowly  dis- 
solved, its  juices  being  utilized  by  the  plant  in  its  nutrition.  (Kerner  and  Oliver.) 

festations  of  irritability  imply  an  expenditure  of  energy 
disproportionate  to  the  force  of  the  irritating  agent. 
The  finger  touching  the  trigger  of  a  gun  exerts  a  very 
slight  pressure,  the  force  of  which  has  no  relation  to  the 
amazing  explosive  force  that  follows  it;  a  small  effort 
turns  the  throttle  of  a  locomotive  by  which  a  heavy  train 
may  be  set  in  motion. 


36  BIOLOGY:  GENERAL  AND  MEDICAL 

These  examples  of  results  disproportionate  to  their 
respective  causes  have  found  their  way  into  many  text- 
books and  unfortunately  confuse  the  student,  for  they 
apply  with  accuracy  only  to  conditions  that  may  be 
compared  to  the  charge  in  the  gun  or  the  vapor  tension 
in  the  boiler.  Thus,  when  we  examine  the  conditions 
found  among  living  things  in  which  such  disproportions 
are  noted,  and  an  explosive  disturbance  follows  what 
seems  to  be  a  trifling  stimulus,  we  find  that  instead  of  a 
simple  reaction  we  have  to  deal  with  some  complicated 
mechanism  in  which  the  transmission  of  the  impulse 
from  the  cell  stimulated  to  many  other  cells,  results  in 
an  effect  which  is  the  sum  of  many  separate  stimulations. 

This  is  the  case  with  the  Mimosa  or  sensitive  plant 
whose  leaves  all  close  when  a  few  pinules  are  touched,  and 
with  Dioncea,  or  the  "Venus'  fly-trap, "  in  which,  when  a 
few  hairs  are  touched,  the  leaf  closes,  entrapping  the 
offending  insect.  The  same  condition  is  found  in  the 
higher  animals  whose  nervous  systems  are  so  coordinated 
as  to  act  reflexly,  the  prick  of  a  pin  or  some  other  slight 
stimulation  sufficing  to  set  in  motion  a  series  of  stimu- 
lations terminating  in  the  involuntary  and  convulsive 
movement  of  a  muscle,  a  group  of  muscles  controlling 
a  member,  or  even  most  of  the  muscles  of  the  body. 

That  such  explosive  reactions  are  exceptional,  and 
that  many  of  the  manifestations  of  irritability  are  appre- 
ciable only  after  the  lapse  of  considerable  time,  and  then 
only  through  slight  changes,  will  soon  become  apparent. 

Stimulations  calling  forth  a  normal  expenditure  of 
energy,  the  loss  of  which  can  be  amply  compensated  for 
by  the  nutritive  function,  may  continue  indefinitely,  as 
is  evidenced  by  the  continuous  operation  of  all  those 
stimuli  that  have  to  do  with  normal  growth  and  function. 
Those  calling  for  excessive  expenditures  of  energy  soon 
exhaust  vitality,  and  throw  the  living  substance  into  an 
inactive  state  known  as  rigidity  or  "tetanus."  If,  after 
the  development  of  this  state,  time  be  given  for  the 
metabolic  functions  to  restore  the  integrity  of  the  proto- 


THE   MANIFESTATIONS   OF   LIFE  37 

plasm,  irritability  returns,  but  if  further  disturbance  is 
effected,  exhaustion  and  death  may  ensue. 

Since  all  the  reactions  of  irritability  are  associated  with 
more  or  less  marked  expenditures  of  energy,  they  all 
result  in  metabolic  disturbances,  and  are  all  associated 
with  chemical  changes. 

Certain  agents — cold,  chloroform,  ether,  chloral,  etc. — 
inhibit  the  irritative  phenomena.  These  are  called 
depressants  and  anesthetics,  and  are  quite  well  known 
experimentally  though  they  are  not  known  to  play  any 
part  in  the  normal  vital  processes. 

Irritable  Reactions  Toward  Stimuli  of  Unknown  Nature. 
— Among  these  are  included  those  stimuli  by  which  the 
development  of  the  organism  is  governed,  and  its  auto- 
matic behavior  determined.  A  superficial  acquaintance 
with  embryology  is  sufficient  to  show  that  under  appro- 
priate conditions  the  germinal  cells  of  plants  and  animals 
invariably  develop  according  to  a  fixed  plan.  Further- 
more, the  developed  animal  behaves  according  to  a  fixed 
plan  of  conduct  inherited  from  its  ancestors.  In  ignor- 
ance of  the  character  of  the  impulses  thus  engaged,  we 
look  upon  them  as  of  physico-chemical  nature. 

Irritable  Reactions  Occasioned  by  Stimuli  of  Known 
Nature. — These  external  stimuli  embrace  all  those  agents 
by  which  the  behavior  of  the  organism  in  its  relations 
to  the  external  world  is  determined.  The  irritable  re- 
sponses to  these  agents  are  frequently  referred  to  as 
tropisms,  and  receive  special  denominations  according  to 
their  respective  qualities. 

THERMOTROPISM  OR  RESPONSE  TO  THERMAL 
STIMULATION. 

No  living  substance  is  indifferent  to  variations  of 
temperature.  It  is  temperature  that  makes  the  con- 
ditions under  which  life  is  possible,  and  it  is  tempera- 
ture that  stimulates  activity  when  the  proper  conditions 
obtain.  Thus,  active  life  is  impossible  without  water  by 


38  BIOLOGY:  GENERAL  AND  MEDICAL 

which  the  protoplasmic  basis  of  the  cells  is  kept  moist 
and  soft,  protoplasmic  currents  established,  and  the 
molecular  interchanges  constituting  metabolism  made 
possible.  If  low  temperature  transform  the  water  to  ice, 
or  if  high  temperature  drive  it  off  in  the  form  'of  vapor, 
life  must  either  cease  or  become  inactive  until  the  essen- 
tial conditions  are  restored. 

Freezing  results  in  disintegration  of  the  protoplasm; 
high  temperatures,  in  coagulation  of  the  delicate  sub- 
stance. Active  vital  manifestations  are  thus  only  pos- 
sible within  limits  marking  the  vital  endurance  of  the 
organisms  observed. 

A  striking  difference  in  temperature  relations  charac- 
terizes the  active  and  passive  states  of  living  matter. 
Thus  many  plants  are  killed  by  frost,  whose  seeds  are 
not  injured  by  any  known  degree  of  cold;  bacteria  that 
are  killed  at  60°  C.,  may,  in  the  spore  stage,  resist  expo- 
sure to  120°  C.  for  a  few  minutes.  This  assumption  of 
the  latent  or  passive  form  subserves  the  useful  purpose 
of  enabling  the  animal  to  escape  the  rigors  of  winter,  the 
excessive  dry  heat  of  the  desert,  and  other  temporarily 
unfavorable  conditions. 

Different  kinds  of  organisms  show  striking  differences 
in  regard  to  temperature  endurance.  In  general  terms 
this  bears  a  direct  relation  to  the  developmental  com- 
plexity of  the  organism.  The  lowest  forms  of  life  some- 
times show  a  temperature  endurance  ranging  over  350° 
C.;  the  highest  forms  may  not  survive  an  actual  change  of 
body  temperature  amounting  to  more  than  5°  C. 

Thus,  the  spores  of  certain  bacilli  may  be  exposed  for 
an  hour  to  the  temperature  of  liquid  hydrogen  (  —  225°  C.) 
and  yet  survive.  When  the  temperature  is  slowly  ele- 
vated, and  the  spores  are  watched,  it  is  found  that  no 
change  occurs  until  6°  C.  is  reached  when  an  occasional 
spore  germinates  and  a  bacillus  emerges.  As  the  tempera- 
ture ascends,  such  of  these  bacilli  as  have  survived  are 
found  to  be  dividing,  at  first  only  at  long  intervals,  then 
with  increasing  frequency  until  when  12.5°  C.  is  reached, 


THE    MANIFESTATIONS    OF   LIFE  39 

division  occurs  every  four  or  five  hours;  at  25°  C.  every 
fifteen  or  twenty  minutes.  Between  this  temperature 
and  40°  C.  there  is  no  essential  change,  but  beyond  it 
multiplication  ceases  or  growth  becomes  modified  so 
that  in  certain  species  spores  are  formed  as  the  bacilli 
cease  to  develop,  in  others  spore  formation  ceases.  When 
the  temperature  ascends  beyond  60°  C.,  no  more  spore- 
free  organisms  can  be  found  alive.  The  spores,  however, 
may  endure  ascending  temperatures  including  exposure 
to  100°  C.  for  an  hour,  and  120°  C.  for  a  few  minutes. 

This  variation  shows  that  there  are  several  tempera- 
tures deserving  special  mention;  the  lowest  at  which  the 
activity  of  the  organism  becomes  manifested,  known  as 
the  minimum,  that  at  which  the  vital  manifestations 
progress  with  greatest  rapidity,  the  optimum,  and  the 
highest  at  which  they  can  be  continued,  the  maximum. 

The  temperature  endurance  of  organisms  differs  in 
many  cases  because  of  special  adaptations.  In  the  case 
of  the  bacteria  it  is  ability  to  enter  upon  a  latent  or  spore 
stage;  in  certain  lowly  animal  forms,  it  is  ability  to  enter 
upon  an  encysted  sta.ge  in  which  the  delicate  protoplasm 
becomes  protected  by  a  dense  capsule;  in  higher  plants 
protection  against  moderate  cold  is  secured,  in  some 
species,  through  the  development  of  a  hairy  covering  by 
which  the  cold  atmosphere  is  kept  away,  in  others  where 
no  such  provision  is  made  and  the  plant  is  killed  by  the 
frosts,  it  prepared  for  the  future  generations  either  by 
the  formation  of  seeds,  some  of  which  can  endure  any 
known  degree  of  cold,  or  by  a  latent  existence  in  the 
form  of  rhizomes  or  bulbs. 

In  the  so-called  "  cold-blooded "  animals,  whose 
temperatures  differ  little  from  those  of  the  surrounding 
atmosphere,  cold  retards  the  metabolic  functions,  and 
heat  accelerates  them.  If  such  animals  can  be  kept  from 
actual  freezing,  they  endure  cold  without  much  harm, 
and  heat  only  injures  them  when  the  accelerated  metab- 
olism becomes  a  source  of  excessive  waste  to  the  cells. 

The    higher,    "warm-blooded,"    animals    and    birds 


40  BIOLOGY:  GENERAL  AND  MEDICAL 

maintain  a  stable  body  temperature  through  heat- 
regulating  mechanisms,  by  which  the  heat  resulting 
from  the  metabolic  processes  is  prevented  from  radiating 
when  the  external  temperature  is  low,  or  radiated  rapidly 
when  it  is  high.  For  these  organisms  any  considerable 
variation  in  the  temperature  of  the  body  itself  is  quickly 
fatal;  so  that,  when  they  are  unable  to  prevent  loss  of 
heat  by  radiation,  through  lack  of  proper  protection  in 
the  way  of  hair  or  feathers,  they  quickly  die  of  cold;  or, 
if  through  any  means  they  are  prevented  from  radiating 
heat,  they  quickly  die  with  an  elevated  body  tempera- 
ture resulting  from  the  accumulation  of  heat. 

The  eggs  of  birds  which  have  no  means  of  maintaining 
or  radiating  heat,  are  affected  by  slight  variations  of 
temperature,  and  afford  striking  examples  of  the  stimu- 
lating as  well  as  the  destructive  effects  of  temperature. 
Thus,  if  a  hen's  egg  be  placed  in  an  incubator  under  favor- 
able conditions,  the  irritability  of  the  germinal  cell  is 
shown  at  about  39°  C.  by  division  and  a  succession  of 
changes  that  will  eventually  result  in  the  development 
of  a  chick.  If,  however,  the  incubator  cool,  or  if  its 
temperature  rise  a  few  degrees  and  remain  so  for  a  few 
hours,  development  ceases  and  the  embryo  dies. 

When  we  come  to  consider  man  we  find  a  high  degree 
of  temperature  susceptibility.  Normally,  the  body 
temperature  is  37°  C.  and  at  this  point  it  is  maintained 
by  complicated  heat-regulating  nervous  mechanisms,  in 
spite  of  external  conditions.  His  cells  are,  however, 
so  susceptible  to  changes  of  temperature  in  the  body 
itself  that  a  variation  of  more  than  one  degree  cannot 
take  place  without  subjective  symptoms;  a  variation  of 
more  than  two  and  one-half  degrees,  without  subjective 
and  objective  symptoms  and  incapacitation  from  the 
usual  activities  of  life;  a  variation  of  three  degrees  with- 
out prostration,  or  of  five  degrees  without  danger  to  life. 

It  is  the  thermal  irritability  of  protoplasm  that  leads 
to  the  varying  vital  manifestations  accompanying  the 
procession  of  the  seasons  as  it  is  seen  in  the  temperate 


THE    MANIFESTATIONS    OF   LIFE  41 

zone,  and  the  general  variation  of  fauna  and  flora  as  we 
see  it  affected  by  latitude  and  altitude. 

In  the  eternal  winter  at  the  earth's  poles,  and  in  the 
eternal  snows  of  high  mountain  altitudes  there  is  no 
life;  on  the  lowlands  near  the  earth's  equator,  where  it  is 
perpetual  summer,  life,  both  vegetable  and  animal,  is 
most  abundant  and  most  active. 

The  most  striking  and  most  beautiful  example  of 
thermotropic  irritability  is  to  be  seen,  however,  in  the 
alternating  summer  and  winter  of  the  temperate  zones. 

During  the  winter  months  when  the  waters  are  locked 
in  ice  and  the  ground  covered  with  snow,  the  landscape 
presents  an  appearance  of  desolation  suggesting  uni- 
versal death.  The  plants  seem  dead,  the  trees  lifeless 
skeletons;  the  insects  and  smaller  animals  have  disap- 
peared; the  migratory  birds  have  flown,  and  only  the  ever- 
green trees,  and  most  hardy  birds  and  mammals  remain. 
If  the  winter  be  exceptionally  severe,  many  of  these 
may  be  killed.  As  the  position  of  the  earth  changes  and 
the  days  lengthen  and  the  warmth  of  the  sun's  rays 
strengthens,  the  conditions  change.  The  snow  and 
ice  melt,  and  in  the  warm  moist  earth  the  roots  and 
seeds  swell  and  the  tender  grass  and  plants  emerge.  The 
trees  soon  spread  their  leafy  canopies,  the  flowers  bloom, 
the  insects  leave  their  hiding  places,  the  birds  return, 
the  animals  creep  from  their  shelters,  and  the  latent 
invisible  life  once  more  returns  to  its  state  of  activity 
and  vigor.  But  see  how  rapid  is  the  accelerating  in- 
fluence of  the  increasing  temperature  in  all  these  changes. 
Upon  a  frosty  spring  morning  in  the  country,  one  notes 
the  greening  grass  and  the  flower  buds  nestling  in  the 
sheltered  places;  an  occasional  insect  clings  feebly  to 
the  stem  of  a  plant  or  crawl  upon  the  ground,  easily 
picked  up,  benumbed  and  stiff;  perhaps  a  snake  has 
curled  upon  a  stone  or  stump  to  catch  the  morning  sun, 
so  stiff  and  sluggish  as  to  fail  to  get  away  in  time  to 
avoid  capture.  Yet,  by  afternoon,  the  magic  of  the  sun's 
warmth  has  effected  a  striking  change:  the  grass  is 


42  BIOLOGY:  GENERAL  AND  MEDICAL 

greener,  the  flower  buds  have  opened,  the  buds  upon 
the  trees  have  doubled  their  size  or  perhaps  burst  into 
tufts  of  tender  leaves,  the  great  bumble-bees  of  spring 
fly  to  and  fro  in  a  business-like  manner,  the  toad  is 
piping  in  the  nearby  pond,  and  the  snake  glides  noise- 
lessly but  quickly  out  of  the  way  as  the  vibrating  earth 
tells  of  your  approach. 

In  these  examples  the  thermic  irritability  is  mani- 
fested by  changes  so  gradual  that  it  is  only  through 
observation  of  their  aggregate  results  that  they  can  be 
detected,  but  examples  are  not  wanting  to  show  that 
upon  those  plants  and  animals  so  constructed  as  to  en- 
able us  to  observe  them  temperature  exerts  an  immedi- 
ate response.  This  is  the  case,  however,  only  when  the 
variations  are  considerable  and  the  changes  sudden.  For 
example,  if  a  Mimosa  be  cautiously  approached  by  either 
a  hot  or  cold  object,  the  greatest  care  being  exercised 
to  avoid  mechanical  contact  with  the  plant,  the  sudden 
change  of  temperature  is  sufficient  to  excite  the 
irritable  cells  and  provoke  closure  of  the  leaves.  It  is 
not  improbable  that  the  irritability  of  living  matter 
is  susceptible  to  the  stimulating  effect  of  any  sudden 
change. 

The  subject  must  not  be  dismissed  without  question- 
ing whether  cold,  as  well  as  warmth,  may  not  have  a 
stimulating  effect.  Cold  has  a  marked  inhibitive  action 
upon  the  vital  processes  and  when  sufficiently  intense 
may  terminate  them;  but  that  it  acts  as  a  stimulus  as 
well  is  not  impossible,  for  certain  bulbs  are  found  to 
grow  more  rapidly,  and  their  plants  to  bloom  more 
quickly  if  they  are  exposed  for  a  short  time,  before 
planting,  to  an  unusually  low  temperature. 

The  diminishing  temperature  of  approaching  winter 
may  be  responsible  for  the  plentiful  growth  of  hair  and 
feathers  in  many  animals  and  birds. 

The  application  of  cold  to  the  human  skin  is  followed 
by  vasomotor  stimulation  resulting  in  contraction  of 
the  peripheral  blood  vessels,  and  the  effect  of  a  cold 


THE    MANIFESTATIONS    OF   LIFE 


43 


wind  upon  the  conjunctiva  is  accompanied  by  lachry- 
mation  or  rapid  secretion  of  tears  in  most  human  beings. 

THIGMOTROPISM  OR  RESPONSE  TO  MECHANICAL 
STIMULATION. 

External  agents  of  indifferent  chemical  and  electrical 
quality,  and  free  from  temperature  variations  to  which 
their  effects  can  be  referred,  excite  varying  reactions  in 
living  organisms  according  to  the  simplicity  or  complexity, 
activity  or  passivity  of 
the  organism  stimulated. 

Animal  and  vegetable 
organisms  differ  in  their 
manifestations,  the  gen- 
eral freedom  of  motion 
among  the  animals  as 
contrasted  with  the  re- 
stricted movements  of 
vegetables  serving  to  ex- 
plain the  indifference  of 
most  vegetable  forms  of 
life  to  the  effects  of  me- 
chanical stimulation. 

Examples  of  thigmotropism  among  plants  are,  how- 
ever, not  wanting.  Thus,  Mimosa  resents  a  very  slight 
mechanical  irritation  by  closing  its  leaves,  and  Dionsea 
closes  its  leaves  to  entrap  its  insect  victim  as  soon  as 
more  than  one  of  the  little  hairs  upon  the  surface  have 
been  disturbed.  An  insect  alighting  upon  a  leaf  of 
Drosera  excites  the  neighboring  tentacles  to  curve  upon 
and  capture  it.  The  tendrils  of  climbing  plants  are 
highly  susceptible  to  the  mechanical  stimulation  of 
bodies  with  which  they  come  into  contact.  "Pfeffer 
found  that  they  were  not  induced  to  coil  by  every  touch, 
but  only  through  contact  with  the  uneven  surface  of 
solid  bodies.  Raindrops,  consequently,  never  act  as  a 
contact  stimulus;  and  even  the  shock  of  a  continual  fall 


FIG.  4. — Leaves  of  Sundew,  a,  Tentacles 
closed  over  captured  prey;  b,  only  half  of 
the  tentacles  closed.  Somewhat  magni- 
fied. (After  Darwin.) 


44  BIOLOGY:  GENERAL  AND  MEDICAL 

of  mercury  produces  no  effect,  though  contact  with  a 
fibre  of  cotton-wool  weighing  only  0.00025  mgr.  is  suffi- 
cient to  stimulate  the  tendril  to  coil. 

The  effect  of  mechanical  stimulation  of  certain  parts 
of  flowers  by  visiting  insects  is  often  shown  in  move- 
ments of  the  stamens,  or  pistil,  by  which  direct  and 
cross  fertilization  of  the  flowers  is  facilitated.  Thus 
the  stamens  of  the  barberry  flowers  are  irritable  and 
when  touched  upon  the  inner  surface  curve  toward  the 
pistil. 


FIQ.  5. — Mimosa.     A.  Position  of  a  leaf  at  rest.     B.  Position  of  contraction 
resulting  from  irritation. 

Among  animals  thigmotropism  or  reaction  to  mechani- 
cal stimulation  is  almost  universal. 

The  amoeba  falling  upon  the  bottom  of  the  pool  in 
which  it  lives  or  touching  the  surface  of  a  glass  slide 
upon  which  it  is  observed  through  a  microscope,  reacts 
by  extending  pseudopods  and  slowly  moves  from  place 
to  place  by  the  streaming  of  its  body  substance.  As  the 
pseudopods  impinge  upon  mechanical  obstacles  they  are 
withdrawn  in  favor  of  .others  whose  progress  is  not 
obstructed.  If  the  active  amoeba  be  touched  with  a 
needle,  it  immediately  withdraws  all  of  the  pseudopods, 


THE    MANIFESTATIONS    OF   LIFE  45 

remains  inactive  for  a  short  time,  then  again  resumes  its 
movement. 

When  Vorticella  is  touched,  a  sudden  and  powerful 
contraction  of  the  pedicle  results,  drawing  the  little 
animal  away  from  the  offending  agent.  A  whole  colony 
of  Carchesium  may  contract  when  one  organism  is 
irritated. 

The  rotifer  with  its  "wheels"  in  motion,  quickly  with- 
draws the  cilia  if  touched,  is  quiescent  for  a  mo- 


FIG.  6. — Vorticella  nebulifera  (Entire  colony  magnified).  C.V,  Contractile 
vacuole.  A  free-swimming  individual  with  two  rings  of  cilia  is  seen  on  the 
right.  When  irritated  the  pedicle  undergoes  a  spiral  shortening  and  the  organ- 
ism is  quickly  drawn  away  from  the  irritant.  (Master-man.) 

ment,  then  again  expands  them  and  continues  to  feed. 
Touched  again,  the  same  effect  may  be  produced,  or 
the  animal  may  let  go  its  hold  and  swim  away  to  try 
a  new  place  where  it  may  feed  undisturbed. 

The  fresh-water  hydra  behaves  in  a  most  interesting 
manner  according  to  the  stimulations  it  receives,  and  we 
soon  perceive  that  the  effects  differ  according  to  the 
quality  of  the  stimulus.  Thus,  when  one  of  the  tentacles 
is  touched  by  some  minute  swimming  animal  or  plant, 


46  BIOLOGY:  GENERAL  AND  MEDICAL 

movements  of  prehension,  associated  with  discharge  of 
the  nettle  threads  may  be  provoked  so  that  the  object 
may  be  paralyzed,  caught,  and  forced  into  the  mouth 
of  the  animal.  If,  however,  the  disturbance  be  greater, 
the  tentacles  may  be  withdrawn  and  the  animal  retracted 
to  a  rounded  mass  scarcely  recognizable  as  a  hydra. 
If  frequently  disturbed,  the  animal  may  let  go  and  move 
off  to  a  position  of  greater  security. 

As  these  primitive  reactions  are  observed,  we  may 
interpret  their  purposes  to  be  primarily  defensive  as  the 
animals  at  rest  are  less  conspicuous  and  may  escape  the 
observation  of  their  enemies.  Soon,  however,  we  come 
to  realize  that  even  in  the  pseudopod  of  the  amoeba  we 
see  the  foreshadowing  of  a  discriminating  power,  based 
upon  the  intensity  and  quality  of  the  impression  received, 
which  becomes  developed  more  and  more  perfectly  until 
the  tactile  sense  of  the  higher  animals  develops. 

So  soon  as  animals  evolve  a  complex  nervous  system 
the  importance  of  thigmotropic  irritability  increases, 
special  organs  being  developed  to  receive  and  transmit 
it  as  the  sense  of  touch,  and  with  it  probably  comes 
the  subjective  appreciation  of  pain  as  well  as  the  peculiar 
coordination  of  nervous  and  muscular  stimuli  known  as 
reflex  action  by  which  involuntary  escape  from  injurious 
stimulations  is  effected. 

CHEMOTROPISM  OR  RESPONSE  TO  CHEMICAL  STIMULATION. 

Substances  capable  of  exciting  a  deleterious  action 
upon  living  substance  are  known  as  poisons. 

Certain  of  them  act  by  virtue  of  their  ability  to  effect  an 
immediate  destructive  combination  with  protoplasm  and 
are  known  as  caustics  and  may  be  subdivided  into  co- 
agulating caustics  by  which  the  protoplasm  is  coagulated, 
and  liquefying  caustics-  by  which  it  is  liquefied. 

Among  the  former  may  be  mentioned  the  metallic 
salts,  acids,  and  some  of  the  essential  oils;  among  the 
latter,  bases  such  as  potash,  soda,  ammonia,  and  arsen- 
ious  acid. 


THE    MANIFESTATIONS    OF   LIFE  47 

Other  poisons  known  to  the  chemists  as  toxins  have  an 
exciting  or  depressing  effect  upon  the  living  substance 
by  which  Life  is  eventually  set  aside  without  visible 
chemical  alteration. 

The  effects  produced  by  poisons  bear  a  direct  relation 
to  their  concentration;  for  many  substances  which  effect 
rapid  destructive  influences  in  strong  solutions  are  not 
only  rendered  harmless,  but  also  useful  by  sufficient 
dilution,  and  many  substances  that  are  commonly  util- 
ized by  the  cells  become  injurious  when  presented  to 
them  in  excess. 

In  dilutions  so  great  that,  recognizable,  harmful  effects 
are  no  longer  to  be  expected,  chemical  agents  may  excite 
no  irritable  manifestations,  or  may  provoke  interesting 
and  important  reactions  that  are  described  as  positive 
or  negative  chemotropism. 

The  chemical  nature  of  many  of  the  substances  by 
which  these  reactions  are  excited  is  unknown,  and  in 
some  of  the  experiments  by  which  chemotropic  effects 
resembling  those  seen  in  nature  are  developed,  we  can- 
not be  sure  that  the  natural  and  experimental  condi- 
tions are  identical  because  identical  effects  are  observed. 

That  chemotropic  influences  play  an  important  func- 
tion in  determining  many  of  the  vital  manifestations  is 
beyond  question,  though  it  is  not  always  possible  to 
pursue  the  investigation  because  information  concern- 
ing the  nature  of  the  chemical  stimuli  is  so  defective. 

Pfeffer  found  that  when  motile  spermatozoids  of  ferns 
are  suspended  in  water  they  are  influenced  by  malic  acid. 
Thus  if  a  capillary  tube  containing  a  dilution  of  this  agent 
be  introduced  into  the  water  containing  them,  the  cells 
swim  toward  it  and  quickly  enter  in  response  to  positive 
chemotropic  influences.  From  such  an  experiment  it 
seems  justifiable  to  conclude  that  the  spermatic  ele- 
ments of  ferns  as  well  as  of  other  cryptogams  find  the 
appropriate  female  elements  to  be  fertilized  by  virtue 
of  chemotropic  influences.  Among  the  higher  plants, 
in  which  pollination  is  effected  by  currents  of  air  which 


48  BIOLOGY:  GENERAL  AND  MEDICAL 

carry  the  pollen  from  the  anthers  to  the  stigma,  or  by 
insects  and  birds  that  carry  the  pollen  grains  from  flower 
to  flower,  chemotropism  is  shown  by  the  inability  of  the 
pollen  grains  of  heterologous  species  and  the  ability  of 
those  of  homologous  species  to  grow  into  the  stigma  and 
descend  to  the  ovules  below. 

The  general  explanation  of  the  ease  with  which  specific 
integrity  is  maintained,  and  hybridization  made  difficult 
in  both  plants  and  animals,  probably  rests  upon  condi- 
tions of  positive  and  negative  chemotropism  existing 
between  the  male  and  female  sexual  elements  of  similar 
and  dissimilar  species. 

Among  animals  whose  ova  and  spermatozoa  are  liber- 
ated freely  in  the  water  in  which  they  live,  the  union  of 
the  cells  must  be  determined  by  chemotropic  influences. 
Among  higher  animals  in  which  the  spermatic  fluid  is 
emitted  into  the  sexual  organs  of  the  female,  it  must 
be  chemotropic  influences  that  govern  the  movements 
of  the  spermatozoa  during  their  progress  toward  the 
ovum,  and  finally  determine  their  entrance  into  it  in- 
stead of  into  other  cells  with  which  they  may  come  into 
contact  during  the  interval. 

Chemotropism  is  specialized  in  the  higher  animals  as 
taste  and  smell. 

A.  Sitotropism  or  Reactions  toward  the  Stimulating 
Influences  of  Food. 

This  can  be  differentiated  with  difficulty  from  simpler 
forms  of  chemotropism.  The  wear  and  tear  of  the  cyto- 
plasm of  the  cells  of  living  organisms  brought  about 
through  their  activities,  makes  it  imperative  that  means 
be  provided  for  reintegration  of  the  impoverished  tissues. 
A  food  supply,  therefore,  becomes  imperative. 

The  simple  character  and  almost  universal  distribu- 
tion of  the  foods  of  plants  make  it  unnecessary  for  them 
to  manifest  sitotropic  activities.  The  more  complex 
food  requirements  of  animal  organisms  determine  that 
the  majority  pursue  or  catch  their  food. 


THE    MANIFESTATIONS    OF    LIFE 


49 


Sitotropic  reactions,  however,  are  observed  among 
lowly  motile  plants,  such  as  bacteria.  Thus  Hertwig 
has  found  that  a  1  per  cent,  solution  of  beef-extract  or  of 
asparagin  has  a  pronounced  attractive  effect  upon 
Bacterium  termo,  Spirillum  undula  and  numerous  other 
organisms.  If  a  fine  capillary  tube  filled  with  such  a  solu- 
tion be  held  in  contact  with  a  drop  of  water  containing 
such  organisms,  a  considerable  mass  of  them  will  be 
found  plugging  the  mouth  of  the  tube  in  from  two  to 
five  minutes,  showing  their  movement  from  the  poorer 
to  the  richer  nutrient  supply  in  response  to  sitotropic 
influences. 


FIG.  7. — Amoeba  ingesting  a  Eugleiia  cyst.     1,  2,  3,  4,  Successive  stages  in  the 
process.     (.Jennings.) 

The  amoeba  gliding  about  takes  up  one  after  another 
objects  suitable  for  food,  as  they  come  within  its  reach. 
If  few  such  be  found,  its  movements  become  more  ac- 
tive and  its  excursions  longer. 

A  hydra  with  tentacles  spread  awaits  the  arrival  of 
its  prey.  If  none  come,  it  changes  its  position  and  tries 
again. 

Caterpillars  hatched  upon  the  trunk  of  a  tree  climb  to 
the  branches  and  reach  the  leaves  upon  which  they  feed. 

As  we  ascend  the  scale  of  life,  and  the  behavior  of  the 
organism  becomes  more  and  more  complicated,  the  sito- 
tropic, hydrotropic,  and  oxytropic  reactions  become  so 


50  BIOLOGY:  GENERAL  AND  MEDICAL 

confused  with  what  are  called  instincts,  at  the  founda- 
tion of  which  they  undoubtedly  lie,  that  they  are  apt  to 
be  lost  sight  of. 

The  struggles  of  a  starving  man  to  secure  food  by 
honest  employment,  by  change  of  locality,  by  solicita- 
tion or  by  theft,  though  rarely  so  regarded,  are  as 
certainly  determined  by  positive  sitotropic — internal 
chemical — conditions  and  by  the  necessity  for  the 
reintegration  experienced  by  his  cells  and  expressed  as 
hunger,  as  is  the  more  simple  behavior  of  the  hydra  or 
the  amceba. 

B.  Hydrotropism,  or  response  to  the  stimulating  influ- 
ence of  water,  is  an  important  form  of  chemotropic  re- 
action the  effects  of  which  are  observable  among  nearly 
all  forms  of  life.  Care  must  be  taken,  however,  to 
separate  such  activity  or  quiescence  as  may  depend 
upon  the  presence  or  absence  of  water  with  favorable 
and  unfavorable  conditions  depending  thereupon,  from 
the  real  hydrotropic  reactions  in  which  the  active  organ- 
ism behaves  peculiarly  in  its  efforts  to  effect  the  best 
utilization  of  available  water. 

As  usual  the  reactions  consist  of  positive  and  negative 
movements. 

The  myxomycetes  show  distinct  positive  hydrotropic 
reactions.  Thus,  if  one  be  placed  upon  a  piece  of  blotting 
paper  so  arranged  as  to  be  dry  at  one  end  and  damp  at 
the  other,  the  amoeboid  movements  of  the  plant  slowly 
bring  it  to  the  moist  end  of  the  paper.  If,  now,  the 
paper  have  its  position  reversed,  so  that  what  was 
formerly  the  dry  end  becomes  the  moist  end  and  vice 
versa,  the  organism  gradually  spreads  its  amoeboid  net- 
work more  and  more  toward  the  moisture  until,  in  the 
course  of  time,  it  has  again  traversed  the  length  of  the 
paper.  This  is  positive  hydrotropism  and  represents  the 
common  form. 

Seedling  plants  usually  arrange  themselves  in  such 
manner  that  the  stems  grow  upward  toward  the  light 
and  heat,  while  the  roots  grow  downward  into  the  soil 


THE    MANIFESTATIONS    OF   LIFE 


51 


in  search  of  moisture.  If,  however,  they  are  artificially 
so  arranged  that  the  source  of  moisture  is  above,  the 
rootlets  turn  up  instead  of  down  in  order  to  obtain  it. 

The  hyphomycetes  or  moulds  which  require  much 
moisture  for  successful  growth,  when  cultivated  in  a 
bottle  containing  a  few  drops  of  liquid,  are  found  to 
conform  in  distribution  to  the  moisture  of  condensation 
upon  the  sides  of  the  glass. 

The  hydrotropic  behavior  bears  some  relation  to  the 


FIG.  8. — "Barentierchen."  This  animalcule  is  capable  of  resisting  the  ill 
effects  of  loss  of  water,  a.  The  active  animal,  b.  The  same  in  the  dry  state 
and  apparently  dead.  When  moistened,  it  absorbs  water  and  resumes  its 
active  form  again.  (After  R.  Hertwig.) 

developmental  stage  of  the  organism;  thus  in  the  myxo- 
mycetes  the  positive  hydrotropic  reactions  continue 
only  during  the  vegetative  stage  and  so  soon  as  the 
fructification  begins,  and  it  is  desirable  to  keep  the 
sporangiophores  dry,  negative  hydropism  begins  and 
the  reactions  are  reversed. 

Loss  of  moisture  and  consequent  inability  to  main- 
tain activity  leads  to  a  variety  of  manifestations  among 
both  animals  and  plants.  Among  the  most  lowly 


52  BIOLOGY:  GENERAL  AND  MEDICAL 

it  usually  results  in  the  formation  of  spores,  or  the 
entrance  of  the  organism  upon  an  encysted  or  latent 
stage.  Among  the  higher  plants  inability  to  assume 
such  forms  and  to  transport  themselves  to  a  new  neigh- 
borhood result  in  death;  in  the  higher  animals  it  results 
in  various  movements  for  the  purpose  of  obtaining  the 
essential  moisture.  Thus  land  crabs  are  compelled  to 
travel  every  day  or  two  to  the  water  for  the  purpose  of 
moistening  the  branchiae,  and  among  still  higher  animals 
the  drying  of  the  pools  and  springs  leads  certain  of  the 
fishes  and  amphibia  to  bury  themselves  deeply  in  the 
mud,  where  they  remain  inactive  until  the  return  of 
rain.  Still  higher  animals  such  as  the  mammalia  may 
be  compelled  to  make  long  periodical  migrations  corre- 
sponding to  the  periods  of  rainfall  and  drought.  Fail- 
ure of  the  water  supply  is  followed  by  death  in  such 
cases. 

C.  Oxytropism,  or  Response  to  the  Stimulating  Effects 
of  Oxygen. 

This  form  of  chemotropism  results  from  the  necessity 
which  all  living  things  experience  with  reference  to 
oxygen,  which  is  essential  to  all  forms  of  life.  For 
most  living  things  the  free  oxygen  of  the  atmosphere  is 
sufficient,  but  a  few  forms  of  life  are  unable  to  make 
use  of  it  and  are  obliged  to  secure  such  oxygen  as  they 
need  by  the  analysis  of  compounds  containing  it.  This 
is  best  exemplified  by  the  anaerobic  bacteria,  which, 
appearing  to  be  overstimulated  by  the  uncombined 
element,  show  no  signs  of  activity  until  it  is  completely 
excluded,  when  they  begin  to  analyze  the  compounds 
from  which  they  may  obtain  it.  The  greater  the  avail- 
able oxygen  in  these  compounds,  the  better  and  more 
actively  the  organisms  multiply,  so  that  solutions  of 
carbohydrates  form  the  best  substratum  for  their 
cultivation. 

When,  on  the  other  hand,  aerobic  bacteria  occur  under 
conditions  which  make  it  difficult  to  secure  sufficient  un- 
combined oxygen  for  their  purposes,  interesting  phenom- 


THE    MANIFESTATIONS    OF   LIFE  53 

ena  are  sometimes  observed;  as,  for  example,  intimate 
association  with  diatomes  in  order  that  they  may  profit 
by  the  oxygen  thrown  off  by  the  little  plants.  Verworn 
observed  a  group  of  bacteria  (Spirochaete  plicatilis) 
surrounding  a  Pinnularia  in  great  numbers,  though 
elsewhere  in  the  preparation  they  were  absent.  The 
bacteria  were  all  at  rest  and  were  present  in  greatest 


FIG.  9. — Mutualism  of  diatome  and  bacteria.     The  bacteria  by  which  the 
diatome  is  surrounded  are  profiting  by  the  oxygen  it  gives  off  in  its  metabolism. 

numbers  near  the  centre  of  the  organism.  Suddenly 
the  diatome  moved  off  a  short  distance,  when  the  bacteria 
left  behind  and  remaining  quiet  a  short  time,  swam  after 
it,  surrounding  it  again  in  a  similar  cluster.  It  was  no 
doubt  the  oxygen  given  off  by  the  plant  that  attracted 
the  bacteria.  In  microscopic  specimens,  prepared  by 
covering  a  drop  of  water  with  a  cover-glass,  Hertwig 
points  out  that  necessity  for  oxygen  eventually  deter- 


54  BIOLOGY:   GENERAL   AND   MEDICAL 

mines  all  the  bacteria,  flagellates,  and  ciliates  to  the 
edges  of  the  glass  or  to  the  air  bubbles  caught  in  the 
liquid. 

Hertwig  also  points  out  that  if  a  plasmodium  of  Aetha- 
lium  septicum  be  enclosed  in  a  cylindrical  vessel  contain- 
ing boiled  water,  the  vessel  closed  with  a  perforated  cork 
and  inverted  over  a  dish  of  fresh  water,  the  plasmodium 
soon  escapes  from  the  boiled  water  to  the  aerated  water 
through  the  opening  in  the  cork. 

If  fishes  be  kept  in  an  aquarium  without  plants  or 
other  means  of  aerating  the  water,  they  will  be  found  to 
behave  in  a  peculiar  manner  as  the  oxygen  becomes 
exhausted.  They  first  rise  to  the  surface  near  which 
they  remain.  As  the  available  oxygen  diminishes,  they 
swim  along  the  surface  with  the  mouth  open  until  a 
globule  of  air  is  secured  and  forced  to  the  back  part  of  the 
mouth  in  order  that  it  may  be  brought  into  contact  with 
the  gills.  By  this  means  asphyxia  may  be  postponed 
for  some  time. 

When  the  higher  animals  are  excluded  from  oxygen  a 
complicated  series  of  nervous  and  muscular  manifesta- 
tions, collectively  known  as  asphyxia,  occurs.  They  are 
chiefly  characterized  by  involuntary  muscular  efforts, 
brought  about  through  excitation  of  the  automatic  res- 
piratory centres,  the  purpose  of  which  is  to  relieve  the 
organism  of  the  accumulated  CO2  and  to  secure  fresh  O. 
As  the  condition  progresses  the  innervation  being  more 
and  more  profoundly  disturbed,  the  movements  become 
more  and  more  violent  until  the  animal  falls  convulsed 
and  exhausted. 

HELIOTROPISM  OR  RESPONSE  TO  PHOTIC  STIMULATION. 

With  but  few  exceptions  living  organisms  are  sensitive 
to  light  and  react  according  to  conditions  not  all  of  which 
are  understood. 

It  is  found  by  experiment  that  the  different  rays  of  the 
solar  spectrum  have  varying  effects  upon  living  organ- 


THE   MANIFESTATIONS   OF   LIFE  55 

isms;  some,  like  the  red  and  yellow  rays,  being  useful; 
others,  like  the  blue  and  violet  rays,  being  prejudicial  in 
action. 

Many  living  organisms  flourish  under  the  direct  rays 
of  a  tropical  sun,  others  are  quickly  killed  by  direct  sun- 
light, still  others  seem  to  find  the  most  favorable  condi- 
tions in  the  perpetual  darkness  of  caverns  or  the  depths 


Fio.  10. — A  seedling  of  the  White  Mustard  in  a  water  culture  which  has  first 
been  illuminated  from  all  sides  and  then  from  one  side  only.  The  stem  is 
turned  toward  the  light,  the  root  away  from  it,  while  the  leaf-blades  are  ex- 
panded at  right  angles  to  the  incident  light.  K,K,  Sheet  of  cork  to  which  the 
seedling  is  attached.  (Strasburger,  Noll,  Schenck,  and  Karsten.) 


of  the  sea.     It  is  thus  possible  to  describe  positive  and 
negative  heliotropism  among  both  plants  and  animals. 

Certain  bacteria  are  highly  susceptible  to  light,  and 
some  species  are  quickly  killed  when  exposed  to  the 
direct  rays  of  the  sun.  Bacteria  and  indeed  most  fungi 
seem  to  flourish  best  in  diffused  light;  a  few  appear  to 
grow  best  in  the  dark,  and  sometimes  certain  functions 


56 


BIOLOGY:  GENERAL  AND  MEDICAL 


of  bacteria,  such  as  the  formation  of  pigment,  take  place 
only  in  the  dark,  as  in  Bacillus  mycoides  roseus. 

To  the  higher  plants,  light  is  indispensable  and  their 
heliotropic  reactions  are  correspondingly  interesting. 

A  familiar  example  of  positive  heliotropism  in  the 
higher  plants  is  found  in  the  sprouting  of  potatoes  in  the 
cellar.  If  the  cellar  be  very  dark,  the  sprouts  are  long, 


Fro.    11. — Mimosa  pudica.     A.   Entire  plant  in  the  daytime  with  leaves  ex- 
panded.     B.  The  same  in  the  position  of  contraction  assumed  at  night.      (Ver- 


slender  and  without  color.  If  there  be  a  distant  window 
from  which  a  dim  light  is  admitted,  the  shoots  on  the 
window  side  are  larger  and  more  vigorous.  If  the  light 
admitted  by  the  window  reach  a  certain  intensity,  small 
leaves  appear  at  the  ends  of  the  sprouts,  and  show  pale 
green  color.  This  growth  of  the  potato  is  entirely  dif- 
ferent from  that  seen  when  the  tuber  is  planted  in  the 


THE    MANIFESTATIONS   OF    LIFE  57 

earth  out  of  doors,  when  a  sturdy  stalk  with  abundant 
dark  green  leaves  assumes  a  vertical  growth. 

Whoever  has  beautified  his  window  with  growing 
plants  must  have  observed  how  regularly  the  leaves  turn 
toward  the  light,  and  how  necessary  it  is  to  turn  the  pot 
around  day  after  day  if  symmetrical  development  of  the 
plant  is  desired.  The  movement  of  the  leaves  depends 
upon  positive  heliotropic  movements  of  the  stems  and 
petioles  by  which  the  leaves  are  kept  at  right  angles  to 
the  incident  light,  thus  exposing  the  entire  upper  surface, 
and  enabling  its  superficial  cells  to  benefit  by  its  influence. 
The  study  of  the  entire  plant  usually  shows  that  the  root 
turns  away  from  the  source  of  light  as  the  leaves  turn 
toward  it.  The  leaves  are  therefore  positively,  the 
roots  negatively,  heliotropic. 

Certain  plants  such  as  Mimosa  partially  or  completely 
close  the  leaves  when  the  sunlight  wanes,  to  open  them 
again  when  it  waxes.  Many  flowers  close  during  the 
night  to  open  again  when  the  morning  sun  strikes  them. 
The  morning-glory  and  dandelion  are  familiar  examples. 

It  is,  of  course,  difficult  to  exclude  the  heat  accompany- 
ing the  sun's  rays  as  a  factor  in  these  movements,  yet 
the  amount  of  heat  in  the  feeble  light  of  the  cellars  in 
which  potatoes  sprout  can  scarcely  be  accompanied  by 
sufficient  heat  to  explain  the  behavior  already  pointed 
out,  and  the  disastrous  effects  of  absence  of  light  in  the 
presence  of  heat  would  indicate  that  it  is  the  light  and 
not  the  heat  that  effects  the  reaction.  Thus,  if  a  well- 
grown,  healthy  plant  be  transferred  to  a  warm  but  dark 
room,  in  spite  of  the  careful  maintenance  of  all  other 
healthy  conditions,  the  plant  soon  sickens  and  the  leaves 
and  flowers  fall  off. 

The  positive  heliotropic  reactions  subserve  a  useful 
purpose  in  bringing  the  essential  organs  of  the  plant  into 
the  sunlight  without  which  its  important  functions  are 
impossible.  Thus,  in  the  dark  no  chlorophyl  can  be 
formed,  and  without  chlorophyl  the  proper  nutrition, 
growth,  and  perfection  of  the  higher  plants  cannot  be 
carried  on. 


58 


BIOLOGY:  GENERAL  AND  MEDICAL 


The  roots  of  plants,  whether  terrestrial  or  aerial,  con- 
tain no  chlorophyl  and,  like  the  fungi,  are  negatively 
heliotropic.  A  majority  of  the  flowers  that  open  in  the 
sunshine  are  more  or  less  colored.  The  negatively 
heliotropic  flowers  that  open  only  at  night  are  invariably 
white. 

Heliotropism  among  animals  is  evidenced  by  a  general 
activity  during  the  daytime  as  compared  with  quiescence 
at  night.  Like  the  flowers,  the  animals  that  enjoy  the 
sunshine  are  apt  to  be  variously  colored;  those  that  live 
in  the  dark  are  apt  to  be  white. 

The  varying  intensity  of  the  sun's  rays  provoke  interest- 
ing temporary  changes  in  the  skins  of  many  animals 


FIG.    12. — Positive  heliotropism  of    Spirographis  spallanzani.     The  source  of 
light  being  on  the  left,  the  polypi  all  turn  in  that  direction.     (Loeb.) 

through  their  stimulating  effects.  Thus  the  skin  of  the 
squid  contains  a  great  number  of  small  chambers  in 
which  an  inky  fluid  is  contained.  These  chambers 
communicate  with  one  another;  and  when  the  animal 
is  upon  a  dark  object  the  superficial  chambers  dilate  to 
receive  added  fluid,  making  the  skin  dark;  when  it  is  upon 
a  pale  surface,  they  contract,  driving  the  fluid  into  the 
deeper  chambers  and  making  the  skin  pale. 

The  chameleon  is  well  known  because  of  its  ready 
change  of  color  as  it  moves  from  object  to  object.  Here 
the  mechanism  is  different  and  the  color  change  depends 
upon  certain  migratory  pigmented  cells  of  the  skin  which 
change  their  positions  with  surprising  rapidity  according 


THE   MANIFESTATIONS   OF   LIFE  59 

to  the  intensity  of  the  light  reflected  upon  the  animal 
by  surrounding  objects.  The  same  change  of  color  is 
seen  among  many  other  reptiles  and  some  batrachia. 

The  attractive  influence  of  a  lamp  upon  nocturnal 
insects  is  a  striking  example  of  positive  heliotropism, 
many  of  the  insects  actually  flying  into  the  light  to  meet 
destruction. 

But  in  the  animal  world  the  most  striking  example  of 
the  irritability  of  the  cells  toward  light  is  shown  in  that 
particular  and  adapted  form  known  as  the  sense  of 
vision,  where  the  light  rays  are  caught  and  intensified  so 
as  to  act  upon  a  special  organ,  the  retina.  Animals 
living  in  perpetual  darkness  are  either  devoid  of  visual 
organs  or  possess  their  rudiments  only. 

The  cells  of  the  higher  plants  contain  peculiar  granules 
known  as  chloroplasts  whose  office  is  the  production  of 
the  chlorophyl  and  other  colored  substances  peculiar  to 
the  leaves  and  flowers.  When  these  are  present  in  the 
deeper  cells  of  the  plants,  into  which  light  cannot  pene- 
trate, or  when  the  plants  are  kept  in  darkness,  they 
develop  into  leucoplasts,  but  if  at  any  time  light  reaches 
them,  a  change  is  effected  through  its  stimulation,  and 
they  become  changed  into  the  chromoplasts  which  give 
the  fruits  and  flowers  their  varied  colors. 

The  effect  of  light  in  the  transformation  of  these  chro- 
moplasts can  be  studied  by  placing  a  photographic  film 
upon  the  surface  of  a  growing  fruit — a  large  apple  an- 
swers the  purpose  well — exposed  to  the  sunlight.  The 
admission  or  exclusion  of  the  sun's  rays,  by  the  denser 
or  lighter  portions  of  the  negative,  results  in  a  photo- 
graphic picture  upon  the  fruit  caused  by  the  formation 
of  perfect  chromoplasts  where  the  light  penetrated  and 
imperfect  formation  where  it  was  withheld.  Interesting 
and  beautiful  pictures  may  thus  be  produced. 

The  effects  of  the  sun's  rays  upon  the  human  skin  are 
well  known,  though  it  is  difficult  to  differentiate  between 
those  attributable  to  heat  and  those  due  to  the  light 
alone.  Exposure  to  the  direct  rays  of  intense  sunlight 


60  BIOLOGY:  GENERAL  AND  MEDICAL 

result  in  injurious  effects,  known  as  sunburn,  characterized 
by  redness  (hyperaemia)  and  the  formation  of  blisters 
(vesication)  and  followed  by  loss  of  the  superficial 
layers  of  'the  epiderm  and  increased  pigmentation  of  the 
deeper  layers,  usually  appearing  as  a  uniform  bronzing 
of  the  skin,  though  in  certain  individuals,  mostly  in 
those  of  fair  complexions,  the  pigment  may  collect  in 
small  dark  spots  (freckles). 

Continued  exposure  to  the  sun  leads  to  deep  bronzing 
and  may  explain  why  the  races  of  men  inhabiting  those 
portions  of  the  earth  where  the  rays  of  the  sun  are  most 
intense  are  uniformly  darker  in  complexion  than  those 
of  less  sunny  climes. 

Concentration  of  the  sun's  rays  by  lenses  results  in 
intensification  of  both  heat  and  light  rays,  the  former 
being  intensely  destructive  to  life.  If,  however,  the 
concentrated  rays  are  passed  through  cooling  apparatus 
so  as  to  be  deprived  of  the  heat,  it  is  found  that  the  light 
rays  are  also  destructive  to  cell  life.  Finsen  has  devised 
a  method  of  destroying  certain  tumors  (squamous  cell 
carcinoma)  by  exposing  them  to  such  concentrated  light 
rays,  the  abnormal  cells  of  the  tumor  seeming  to  be  less 
able  to  endure  their  effects  than  those  of  the  normal 
tissue  cells  among  which  they  grow. 

GALVANOTROPISM   OR  RESPONSE  TO   ELECTRICAL 
STIMULI. 

Electrical  currents  of  high  intensity  are  destructive 
to  all  forms  of  life  through  chemical  and  physical  altera- 
tions effected  in  the  protoplasm.  Currents  too  mild  to 
be  destructive  influence  living  matter  but  slightly,  and 
little  evidence  is  at  hand  to  show  that  stimuli  of  electrical 
nature  play  any  important  role  in  the  vital  processes. 

Plants  seem  to  be  far  less  sensitive  than  animals  to 
the  effects  of  electric  currents.  The  phanerogames,  in- 
deed, show  no  visible  electrotropic  reactions,  though  the 
cells  when  examined  microscopically,  as  in  the  hairs  of 


THE   MANIFESTATIONS   OF   LIFE 


61 


Tradescantia,  show  a  disturbance  by  which  the  delicate 
protoplasm  is  collected  into  nodular  masses. 

When  amoeba  and  leucocytes  are  subjected  to  the 
irritation  of  galvanic  currents  passed  through  the  fluid 
in  which  they  are  suspended,  they  draw  in  their  pseudo- 
pods,  cease  amoeboid  movement,  and  may  even  suspend 
the  cytoplasmic  circulation.  If  the  current  be  of  mild 
intensity,  its  effect  soon  wears  off  and  activities  begin 


Fio.  13. — Result  of  electric  stimulation  of  plant  protoplasm  as  shown  in  the 
cells  of  the  hairs  upon  Tradescantia  leaves.  A,  Quietly  streaming  cytoplasm; 
B,  changes  produced  by  the  passage  of  an  electric  current,  the  cytoplasm  be- 
ing gathered  into  small  globular  masses  at  c  and  d.  (After  Kuhne.) 

again.     If,  however,  its  intensity  be  greater,  disintegra- 
tion of  the  protoplasm  follows. 

When  water  containing  paramcecia  is  subjected  to  a 
constant  current  of  mild  intensity  passed  from  one  side 
to  the  other,  the  organisms  abandon  the  positive  (anode) 
pole  and  collect  at  the  negative  (kathode)  pole. 
.  When  a  single  organism  subjected  to  a  galvanic 
current  is  examined  microscopically  it  is  found  that 


62 


BIOLOGY:  GENERAL  AND  MEDICAL 


the  effect  of  the  current  is  to  alter  the  position  of  the 
cilia  at  the  kathodal  end  or  side,  so  that  the  organism 
changes  its  direction  and  swims  backwards. 

If  the  experiment  is  performed  with  water  containing 
both  ciliates  and  flagellates,  the  electrotropic  reaction 
is  found  to  be  different  for  the  two  kinds  of  organisms. 


FIG.  14. — Electrotropic  reaction  of  Paramcecium.  The  lower  figure  shows  the 
mode  of  applying  the  electric  current,  the  upper  a  microscopic  field  showing  the 
migration  of  the  organisms  from  the  +  to  the  —  pole.  (Verworn.) 

The  ciliates,  as  has  been  shown,  tend  toward  the  kathode, 
but  the  flagellates  tend  toward  the  anode. 

The  most  active  responses  to  electrical  stimuli  appear 
in  animals  possessed  of  a  nervous  system,  by  which  the 
activity  of  other  parts  of  the  body  is  dominated.  All 
nervous  tissue  seems  to  be  highly  susceptible  to  elec- 


THE    MANIFESTATIONS   OF   LIFE  63 

trical  conduction  and  stimulation,  so  that  the  applica- 
tion of  an  electrode  to  the  central  end  of  a  motor  nerve  is 
followed  by  immediate  muscular  contraction;  to  the 
central  end  of  a  secretory  nerve  by  secretion  on  the  part 
of  the  glands  governed  by  the  nerve,  and  to  the  periph- 
eral end  of  a  sensory  nerve  by  painful  sensations. 

The  facility  with  which  electric  currents  are  trans- 
mitted by  the  nerves  has  led  to  the  assumption  by  many 
that  electricity  and  nerve  force  are  identical. 

In  the  transmission  of  electric  currents  along  the 
nerve  fibres  there  is  a  difference  in  degree  only  between 
anodal  and  kathodal  stimulation. 

Loeb  transmitted  an  electric  current  through  a  trough 
of  water  containing  an  Amblystoma  and  found  that  a 
secretion  of  sticky  white  mucus  appeared  upon  the 
skin  wherever  it  was  struck  by  the  current  waves  ema- 
nating from  the  anode. 

Currents  of  considerable  intensity  produce  cytolysis 
or  disintegration  of  the  protoplasm  probably  by  trans- 
formation of  the  electrolites  of  the  contained  salts. 
Kiihne  found  that  when  a  rhizopod  known  as  Actino- 
sphserium  was  subjected  for  some  time  to  a  constant 
current,  it  began  to  disintegrate  upon  the  anodal  side. 

Currents  of  high  intensity  passed  through  the  higher 
animals  cause  death  from  destruction  of  the  nervous 
system.  It  is  in  this  way  that  men  are  killed  by  con- 
tact, with  "live"  trolley  wires  and  by  electrocution. 

GEOTROPISM  OR  RESPONSE  TOWARD  THE  FORCE  OF 
GRAVITY. 

The  effect  of  gravity  upon  living  things  is  pronounced 
and  occasions  a  variety  of  reactions.  It  seems  to  be 
more  clearly  manifested  among  the  vegetable  than 
among-  the  animal  organisms  because  of  the  greater 
freedom  of  movement  of  the  latter,  but  gravitation 
reactions,  such  as  maintaining  the  equilibrium,  are  to  be 
found  among  the  very  highest  animals. 


64  BIOLOGY:  GENERAL  AND  MEDICAL 

Among  the  lowest  animals  and  plants,  especially 
those  that  are  free  and  motile,  geotropic  reactions  are 
either  undetermined  or  vague. 

Among  the  fungi,  however,  with  increasing  complexity 
of  structure,  there  is  an  increasing  disposition  toward  a 
definite  adjustment  of  the  organism  with  reference  to  the 
earth's  surface.  Thus  among  the  moulds,  Aspergillus 
and  Penicillium  most  commonly  direct  their  sporangia 
upward,  and  among  hyphomycetes  in  general  the  aerial 
hyphse  grow  perpendicularly  to  the  plane  of  the  earth's 
surface. 

Among  the  Basidiomycetes,  the  Polyphorea?  and 
Agaricinese,  which  include  the  mushroom  and  toad- 
stool-like organisms,  tend  toward  a  perpendicular  posi- 
tion, the  pileus  spreading  in  a  plane  corresponding  to 
that  of  the  earth's  surface. 

The  general  tendency  of  the  higher  cryptogams  is  to 
maintain  a  line  of  growth  perpendicular  to  the  plane 
of  the  earth's  surface. 

The  same  general  tendency  pervades  pretty  much 
the  whole  group  of  phanerogams. 

In  considering  the  geotropic  reactions  of  plants,  it 
becomes  necessary  to  speak  of  positive,  negative,  and 
lateral  geotropism  and  to  make  brief  mention  of  diageo- 
tropism. 

In  such  plants  as  show  typical  geotropic  reactions, 
the  stem  which  rises  vertically,  that  is,  perpendicularly 
to  the  plane  of  the  earth's  surface,  is  negatively  geotropic; 
the  branches  that  extend  from  it  in  a  plane  more  or  less 
parallel  with  the  earth's  surface,  diageotropic,  and  the 
tap-root,  that  descends  perpendicularly  to  the  earth's 
surface,  positively  geotropic. 

This  behavior  takes  place  regardless  of  the  sources 
of  light  and  heat  and  in  obedience  to  the  force  of  gravity 
alone. 

Knight  found  that  if  germinating  seeds  were  fastened 
to  a  rapidly  revolving  wheel  moving  in  a  vertical  plane, 
by  which  the  force  of  gravity  was  set  aside,  the  direction 


THE   MANIFESTATIONS   OF   LIFE 


65 


of  growth  obeyed  the  laws  of  centrifugal  force  and  the 
shoot  grew  toward  the  centre  of  attraction  and  its  root 
away  from  it.  When  the  wheel  was  revolved  in  a  hori- 
zontal plane,  the  force  of  gravity  not  being  overcome, 
the  plant  being  subjected  simultaneously  to  both  centrif- 
ugal force  and  that  of  gravitation,  took  an  intermediate 
position,  directing  the  shoot  upward  and  toward  the 
centre,  the  root  downward  and  away  from  it. 

It  is  a  common  observation  that  plants   that   have 


FIG.  15- — The  movements  by  which  a  flower  of  Aconitum  napellus  regains  its 
proper  position  when  the  axis  bearing  it  (s)  is  inverted.  I.  Inverted  position; 
II.  position  resulting  from  geotropism,  the  flower  facing  the  parent  axis;  III. 
flower  again  facing  outward,  after  the  exotropic  movement.  (Strasburger, 
Noll,  Schenck  and  Karsten.) 

made  a  false  start,  through  accidental  circumstance 
or  intentional  interference,  adjust  themselves  to  the 
geotropic  influences  by  certain  curvatures  that  result 
from  increased  growth  of  one  side  and  retarded  growth 
of  the  opposite  side,  the  region  of  greatest  growth  being, 
in  general,  that  of  greatest  curvature.  This  applies 
both  to  the  negatively  geotropic  stems  and  the  positively 
geotropic  roots.  As  soon  as  the  unequal  growth  succeeds 
in  establishing  the  upright  position,  it  ceases  and  sym- 
metrical growth  progresses. 

•    Lateral  geotropism  is  best   exemplified  in   climbing 
5 


66  BIOLOGY:  GENERAL  AND  MEDICAL 

plants  whose  stems  twine  about  upright  supports.  The 
lateral  unequal  growth  is  supposed  to  depend  upon 
geotropic  stimulation  of  the  cells  in  one  lateral  plane 
with  resulting  horizontal  curvatures. 

In  all  of  these  examples  the  geotropic  influences  are 
manifested  through  the  combined  effects  of  minute 
changes  in  many  cells,  the  changes  in  the  individual 
cells  being  too  slight  to  be  recognized. 

Among  animals,  as  has  been  pointed  out,  geotropic 
reactions  are  less  easy  to  define.  With  few  exceptions, 
however,  all  animals  tend  to  maintain  a  definite  position 
with  reference  to  the  earth's  surface  and  axis,  which,  of 
course,  is  a  form  of  geotropism.  Plant-like  animals  are 
best  adapted  to  the  purpose  of  demonstration  and  an 
examination  of  the  members  of  Ccelenterata,  especially 
the  Hydrozoa  and  Actinozoa,  reveal  a  number  of  forms 
whose  geotropic  reactions  are  pronounced. 

These  animals  are  characterized  by  a  cylindrical  body 
with  a  stolon  or  foot  at  one  end  and  a  circle  of  tentacles 
at  the  other.  In  general  they  assume  a  position  with 
the  stolon  down  and  the  tentacles  up.  One  of  these 
hydroids,  Tubularia  mesembryantheum,  was  experi- 
mented upon  by  Allman  who  found  that  if  the  polyp 
was  cut  from  the  stem,  a  new  polyp  regenerated;  if  the 
stolon  was  cut  from  the  other  end  a  new  stolon  developed. 
If  both  were  amputated,  a  new  polyp  was  reproduced 
and  a  new  stolon  reproduced,  but  always  at  those  ends 
from  which  they  had  respectively  been  removed. 

Loeb  endeavored  to  find  out  whether  this  was  due  to 
the  animal  being,  as  Allman  expressed  it,  polarized,  and 
discovered  that  when  both  extremities  were  amputated 
and  the  oral  or  polyp  end  embedded  in  sand,  while  the 
upward  directed  aboral  or  stolon  end  was  surrounded 
on  all  sides  by  water,  a  polyp  instead  of  a  stolon  in- 
variably developed  at  the  aboral  end. 

While  not  attributed  to  geotropic  influences  by  Loeb, 
it  would  seem  as  if  this  peculiarity  might  have  some 
reference  to  them. 


THE    MANIFESTATIONS    OF   LIFE  67 

Loeb  points  out  that  certain  Holothurians  tend  to 
creep  vertically  upward  when  placed  upon  a  plane 
surface.  If  the  plane  be  slowly  turned  so  that  the 
position  is  inverted,  the  animal  remains  quiet  until 
accommodated  to  the  new  order,  and  then  again  be- 
gins to  creep  upward.  This  is  an  example  of  negative 
geotropism. 

Among  the  higher  animals  the  disposition  to  assume 
definite  positions  with  relation  to  gravitation  is  even 
more  pronounced.  The  mechanism  by  which  it  is 
accomplished  is  complicated,  being  partly  voluntary 
and  partly  reflex,  and  accomplished  through  visual 
and  tactile  impressions,  as  well  as  through  the  semicircu- 
lar canals  of  the  internal  ear  which  are  supposed  to  be 
equilibrating  organs. 

Inversion  of  the  higher  animals,  or  the  forced  assump- 
tion of  any  abnormal  position,  is  followed  by  intense 
anxiety  to  resume  the  normal,  but  so  many  factors  co- 
operate to  produce  the  effects  that  it  becomes  extremely 
difficult  to  determine  in  how  far  they  are  geotropic 
in  character. 

CONDUCTIVITY. 

By  conductivity  is  meant  the  conduction  or  trans- 
mission of  any  stimulus  from  the  part  immediately 
irritated  or  stimulated  to  others  more  or  less  remote. 

Conductivity  is  almost  as  widely  distributed  a  property 
of  living  substance  as  is  the  irritability  upon  which  it 
depends. 

As  irritability  was  difficult  to  determine  in  the  absence 
of  immediate  response,  so  it  is  only  possible  to  deter- 
mine the  extent  of  conduction  in  cases  in  which  it  leads 
to  visible  effects. 

In  the  behavior  of  the  plasmodia  of  the  slime  moulds 
we  find  evidences  that  the  effect  of  stimulation  is  exerted 
upon  the  whole,  not  upon  that  particular  portion  stimu- 
lated. Thus,  there  must  be  transmission  of  the  stimu- 


68  BIOLOGY   GENERAL   AND  MEDICAL 


FIG.  16. — Sundew  (Drosera  rotundifolia) .  Each  leaf  is  covered  with  tentacles 
whose  function  is  to  catch  the  insects  upon  which  the  plant  preys.  Each  ten- 
tacle is  covered  with  a  sticky  secretion  which  detains  the  victim  until  neighbor- 
ing tentacles  are  able  to  curve  over  it  and  completely  invest  it.  The  insect  ia 
then  smothered,  and  digested  by  enzymic  secretions,  its  substance  aiding  the 
nutrition  of  the  plant.  (From  Bergen  and  Davis's  "Principles  of  Botany," 
Ginn  &  Co.,  publishers.) 


THE   MANIFESTATIONS   OF   LIFE  60 

lation  from  the  part  stimulated  to  all  parts  of  the 
organism. 

Among  such  of  the  higher  plants  as  show  response  to 
stimulation  we  find  the  effects  to  result  from  the  sum 
of  the  reactions  taking  place  in  numbers  of  cells  through 
irritable  impulses  transmitted  from  cell  to  cell  from 
the  point  of  primary  stimulation.  For  example,  when 
the  hairs  upon  the  expanded  leaf  of  Dionoea  are  irritated, 
the  leaf  closes  through  changes  effected  in  cells  remote 
from  the  point  of  stimulation.  Indeed,  so  many  cells 
are  affected  and  the  effect  of  their  united  activity  so 
pronounced  as  to  lead  to  a  result  strikingly  dispropor- 
tionate to  the  intensity  of  the  stimulation.  The  same 
general  principle  applies  to  Drosera  and  other  insect- 
catching  plants.  A  few  tentacles  being  stimulated  by 
the  insect,  many  bend  over  and  assist  in  catching  it. 

Another  example  in '  which  still  more  widespread 
conduction  of  the  impulse  from  cell  to  cell  is  found 
in  Mimosa,  for  when  a  single  pinnule  of  one  of  the  leaves 
is  actively  stimulated,  the  whole  leaf,  or  the  whole 
branch,  or,  indeed,  the  whole  plant,  may  be  so  disturbed 
as  to  close  its  leaves.  Here  we  see  indubitable  evidence 
that  the  impulse  to  react  passes  from  cell  to  cell  along 
the  pinnules,  petioles,  branches,  and  stems. 

Among  animals  conductivity  is  more  easily  demon- 
strated because  of  the  generally  greater  activity  of  ani- 
mal organisms.  In  plants  the  signs  of  conductivity 
and  irritability  are  most  evident  among  the  lowest  forms, 
but  in  animals  they  are  most  obvious  and  best  developed 
among  the  highest  forms. 

When  a  moving  amoeba  is  irritated  in  such  manner  that 
a  single  pseudopod  is  affected,  all  of  the  pseudopods 
are  drawn  in,  the  spherical  shape  is  assumed,  and  the 
animal  remains  quiet  for  a  time.  When  the  delicate 
pseudopods  of  any  of  the  radiolaria  are  touched,  they 
may  be  withdrawn,  or  all  of  the  pseudopods  may  be  with- 
drawn as  in  amoeba.  In  these  illustrations  it  will  be 
seen  that  a  disturbance  at  one  point  is  transmitted 


70 


BIOLOGY:  GENERAL  AND  MEDICAL 


throughout  the  entire  cell,  all  parts  of  which  are  affected 
by  the  disturbance. 

When  vorticella  is  touched,  the  irritation  is  immedi- 
ately transmitted  to  the  pedicle,  which  contracts  suddenly 
and  violently,  withdrawing  the  animal  from  the  harmful 
influence.  In  Carchesium,  disturbance  of  one  individual 
may  result  in  contraction  of  all  the  stalks  so  that  the 
whole  colony  is  withdrawn  from  the  source  of  stimulation. 

When  a  tentacle  of  one  of  the  hydroids  discovers  some 


FIG.  17. — Vorticella.    a,  Extended;  b,  contracted;  c,  section  of  the  stalk  showing 
the  elongate   contractile   (muscle)    fibre   in   its   interior.     (Verworn.) 


object  useful  for  food,  the  stimulation  is  quickly  imparted 
to  other  tentacles  in  order  that  it  be  firmly  grasped  and 
brought  to  the  oral  opening. 

In  the  higher  animals  whose  movements  must  be 
precise  and  well-coordinated,  this  simple  transmission  of 
irritable  reaction  from  cell  to  cell  is  inadequate  because 
too  slow  to  meet  the  requirements,  and  is  replaced  by 
specialized  cells,  nerve  fibres,  greatly  elongated  in  form, 
and  able  to  establish  immediate  communication  between 


THE   MANIFESTATIONS   OF   LIFE  71 

the  various  parts  of  the  organism.  Among  these  highly 
specialized  conducting  tissues  we  find  terminal  endings 
by  which  the  impulses  or  stimuli  are  received,  fibres  by 
which  they  are  transmitted,  and  ganglionic  cells  by  which 
they  are  received  and  systematically  redistributed,  fibres 
by  which  the  new  impusles  are  further  transmitted,  and 
finally  nervous  terminations  by  which  the  final  stimuli 
are  imparted  to  the  particular  cell  groups  for  which  they 
are  intended. 

MOTION. 

Motion  and  locomotion  are  widespread  vital  manifes- 
tations, and  are  among  those  for  which  we  first  seek  in 
endeavoring  to  decide  whether  or  not  some  newly  dis- 
covered object  is  living  or  not  Iving.  If  it  move  through 
activities  resident  within  itself,  we  are  satisfied  that  it  is 
alive,  but  if  through  obedience  to. external  forces,  further 
investigation  of  its  properties  and  consideration  of  its 
other  manifestations  become  necessary. 

It  is  taught  by  some  biologists  that  some  movement 
is  to  be  found  in  every  living  thing,  but  it  has  already 
been  pointed  out  that  life  exists  in  both  active  or  kinetic 
and  latent  or  potential  forms  and  that  what  applies  to 
the  former  may  not  apply  to  the  latter.  While,  there- 
fore, it  is  quite  true  that  some  form  of  motion  exists  in 
all  active  life,  it  is  difficult  to  imagine  it  in  such  latent 
forms  of  life  as  the  spores  of  fungi  and  the  dry  seeds  of 
plants. 

Motion  and  locomotion  must  not  be  confused.  Many 
living  things  are  in  constant  motion,  to  which  locomotion 
is  impossible. 

One  must  also  avoid  the  hasty  conclusion  that  no 
movement  is  in  progress  because  none  can  be  seen.  In 
reality  the  motion  that  can  be  seen  is  greatly  outweighed 
in  importance  by  the  motion  that  cannot  be  seen.  Thus 
one  sits  quietly  in  a  chair  for  a  time,  then  rising,  takes  a 
turn  about  the  room  and  reseats  himself.  To  the  un- 
informed, it  may  appear  as  if  the  only  movements  made 


72  BIOLOGY:  GENERAL  AND  MEDICAL 

comprised  the  little  walk  about  the  room,  yet  all  the 
time  he  sat  quietly  in  the  chair,  there  were  going  on  within 
his  body  a  great  number  of  invisible  movements,  for  the 
heart  was  continually  beating,  the  blood  and  lymph  were 
continually  circulating,  the  function  of  digestion  was 
probably  in  progress  with  its  involved  movements  of 
secretion,  muscular  contraction,  absorption,  excretion, 
etc.,  and  during  all  this  time  the  cells  of  the  entire  body 
were  more  or  less  actively  nourishing  themselves,  their 
cytoplasm  continually  flowing  to  and  fro  in  rhythmical 
currents. 

We  are  accustomed  to  think  of  plants  as  motionless 
things,  yet  to  one  who  has  come  to  the  realization  of 
what  plant  life  really  means  a  growing  plant  is  full  of 
energy.  There  are  currents  of  sap  ascending  the  stems 
and  flowing  into  the  leaves,  bringing  to  these  great  cellu- 
lar laboratories  the  nutritious  substances  absorbed  by 
the  rootlets  from  the  soil.  As  the  sap  comes  in  it  is 
absorbed  by  the  active  cells  which  work  it  over,  extract- 
ing useful  substances,  manufacturing  new  compounds, 
and  then  sending  these  away  in  currents,  sometimes  to 
the  flowers,  sometimes  to  the  seeds,  and  sometimes  to 
the  roots  where  the  newly  prepared  compounds,  changed 
or  unchanged,  are  stored  up  for  future  needs.  Add  to 
this  the  continuous  accession  of  new  cells  by  multiplica- 
tion of  those  already  present,  the  necessary  gaseous  and 
nutritional  changes  by  which  the  substance  of  the  cells 
is  itself  kept  alive,  and  we  find  the  plant,  seeming  all  the 
while  to  be  inactive,  full  of  life  and  motion. 

Cytoplasmic  Circulation. — This  form  of  movement  is  in 
constant  progress  in  every  active  cell.  It  is  most  active 
and  most  easily  observed  in  young  cells,  especially 
vegetable  cells,  such  as  are  found  in  the  hairs  of  Trad- 
escansia,  which  are  very  soft  and  moist,  and  in  the 
amceba.  It  consists  in  a  regular  flowing  motion  of  the 
cytoplasm  within  the  confines  of  the  cell  and  subserves 
the  double  purpose  of  affording  all  portions  of  the  pro- 
toplasm an  opportunity  of  coming  into  contact  with 


THE    MANIFESTATIONS    OF   LIFE 


73 


the  contained  nutritious  matter,  and  of  enabling  the 
nutritious  matter  to  be  acted 
upon  and  transformed 
by  the  enzymic  substances 
contained  in  the  cytoplasm. 
It  is  probably  indispensable 
to  the  phenomena  of  molec- 
ular exchange  constituting 
metabolism,  and  may  there- 
fore be  presumed  to  be  in 
uninterrupted  progress  in 
every  active  living  cell.  The 
more  active  the  cell,  the 
greater  the  need  of  such  cir- 
culatory movements  and  the 
more  active  they  become. 

In  the  higher  protozoa  the 
cytoplasmic  circulation  is 
facilitated  by  certain  organs 
known  as  contractile  vacu- 
oles.  Thus  if  one  of  the 
large  vacuoles  in  a  Para- 
mcecium  be  carefully  watch- 
ed it  will  be  found  to  undergo 
a  rhythmical  change  of  place, 
becoming  rapidly  smaller 
and  disappearing  where  first 
seen,  to  appear  and  grow 
correspondingly  larger  at  a 
new  situation.  After  a  given 
interval  contraction  again 
takes  place  and  the  vacuole 
reappears  in  the  original  situ- 
ation as  it  disappears  from 

rr\i  FIG.  18. — Two  cells  and  a  part  of  a 

its  recent  one.    These  vacu-  third  from  the  tip  of  a  ,leapr  of  a 

Oles    thus   perform  a    f Unc-    stonewort,  showing    rotation   of    the 

-tion    suggestive  of  a  primi-  v™^»*™  ™  the  direction  of  the 

.          °    .  .  *  arrows.     (Sedgvnck  and  Wilson.) 

tive  heart,  keeping  the  cyto- 
plasm constantly  agitated  by  currents  of  circulating  fluid. 


74 


BIOLOGY:  GENERAL  AND  MEDICAL 


Amceboid  Movement. — This  is  movement  of  a  kind 
best  exemplified  by  the  Amoeba,  and  is  a  primitive  form 
of  locomotion.  It  consists  in  a  peculiar  flowing  of  the 
cytoplasm,  but  instead  of  being  confined  to  the  bound- 


FIG.  20. 

FIG.  19. — Paramaecium  caudatum,  from  the  ventral  side,  showing  the  vestibule 
en  face;  arrows  inside  the  body  indicate  the  direction  of  protoplasmic  currents; 
those  outside,  the  direction  of  water  currents  caused  by  the  cilia,  c.v,  Con- 
tractile vacuoles;  f.v,  food  vaciioles;  w.v,  water  vacuoles;  m,  mouth;  mac, 
macronucleus;  mic,  micronucleus;  ce,  rcsophagus;  v,  vestibule.  The  anterior 
end  is  directed  upward.  (Sedgunck  and  Wilson.) 

FIG.  120. — -Amoeba  proteus:  n,  nucleus;  c.v,  contractile  vacuole;  N,  nutrient 
material  in  process  of  digestion;  p,  pseudopod;  en,  endosarc;  ek,  ectosarc. 
(From  It.  Hertwig.) 

aries  of  the  cell,  it  results  in  an  extension  of  these  bound- 
aries in  the  form  of  what  are  called  pseudopodia. 

Jennings  has  described  the  movement  as  comparable 
to  rolling.  The  upper  surface  continually  passing 
forward  and  rolling  under  at  the  anterior  end  so  as  to 
form  the  lower  surface.  This  causes  the  moving  amceba 


THE- MANIFESTATIONS   OP  LIFE  75 

to  appear  to  have  its  substance  continually  flowing  for- 
ward in  the  direction  of  progress. 

As  the  moving  amoeba  is  watched  it  seems  to  be  un- 
certain as  to  the  best  course  to  pursue  so  that  the  an- 
terior edge  is  not  uniformly  extended,  but  commonly 
flows  out  into  elongate  rounded  processes,  the  pseudo- 
podia,  one  of  which  becomes  larger  and  larger  as  the 
cytoplasm  flows  into  it,  while  the  remainder  are  gradu- 
ally withdrawn. 

Progress  is  effected  with  great  slowness,  and  through 
an  unending  series  of  changes  in  the  shape  of  the 
organism. 


FIG.  21. — Diagram  of  the  movements  in  a  progressing  amoeba  in  side  view. 
A,  anterior  end;  P,  posterior  end.  The  large  arrow  above  shows  the  direction 
of  locomotion;  the  other  arrows  show  the  direction  of  the  protoplasmic  currents, 
the  longer  ones  representing  more  rapid  currents.  From  a  to  x  the  surface  is 
attached  and  at  rest.  From  x  to  y  the  protoplasm  is  not  attached  and  is  slowly 
contracting,  on  the  lower  surface  as  well  as  above,  a,  b,  c,  successive  positions 
occupied  by  the  anterior  edge.  As  the  animal  rolls  forward,  it  comes  later  to 
occupy  the  position  shown  by  the  broken  outline.  (Jennings.) 

Locomotion  is  quite  free  in  the  amceba,  but  cells  may 
lack  locomotory  power  and  still  be  amoeboid;  i.e.,  capable 
of  changing  their  shape.  Thus  many  of  the  radiolaria 
and  foraminifera  being  inclosed  in  mineral  shells  can- 
not move  from  place  to  place,  though  they  commonly 
extend  pseudopodia,  which  are  sometimes  extremely 
long  and  delicate,  through  the  minute  openings  of  their 
shells. 

The  cells  of  the  more  simple  metazoa  retain  a  limited 
amoeboid  movement,  and  in  the  highest  animals  amoe- 
boid cells  may  still  be  found.  The  best  known  of  these 
.is  the  white  blood  corpuscle  (polymorphonuclear). 


76 


BIOLOGY:  GENERAL  AND  MEDICAL 


Under  abnormal  conditions  many  of  the  fixed  cells 
of  the  higher  animals  show  that  they  retain  the  amreboid 
movement  to  a  limited  extent. 

Ciliate  and  Flagellate  Movements. — Cilia  are  minute 
short  hair-like  processes  with  which  many  cells  are 
provided;  flagella,  larger,  coarser,  whip-like  processes. 
Between  the  two  there  is  no  sharp  line  of  distinction, 
and  the  numerous  cilia  of  the  bacteria  are  universally 
known  as  flagella. 


FIG.  22. — Amceba  verrucosa  coiling  up  and  ingesting  a  filament  of  oscillaria, 
After  Rhumhler  (1898).  The  letters  a  to  g,  show  successive  stages  in  the 
process.  (Jennings.) 

Cilia  and  flagella  are  specialized  processes  of  the  cell 
substance,  composed  of  hyaloplasm.  They  are  analo- 
gous to  pseudopods,  but  differ  from  them  in  their  more 
uniform  and  delicate  structure  and  in  being  permanent 
instead  of  temporary  structures.  They  can  be  made 
use  of  to  assist  in  the  classification  of  many  organisms. 

Cilia  are  utilized  for  the  double  purpose  of  motion 
and  locomotion.  Thus  many  of  the  infusoria  are 
covered  with  minute  hair-like  cilia  of  uniform  size, 
whose  synchronous  vibrations  propel  the  organisms 


THE    MANIFESTATIONS    OF   LIFE 


77 


at  a  fair  rate  of  speed  through  the  fluids  in  which  they 
live.  These  are  locomotory  in  function.  It  is  interest- 
ing to  observe  that  they  may  vibrate  synchronously 
in  a  given  direction,  or  vibrate  on  one  side  of  the  body 
more  rapidly  than  on  the  other 
so  that  the  direction  may  be 
changed,  or  may  reverse  their 
direction  so  that  the  organism 
may  move  backwards.  When 
organisms  like  Paramcecium 
conjoin,  their  cilia  vibrate 
synchronously  so  that  they 
easily  swim  along  together. 
As  has  been  shown  in  speak- 
ing of  galvanotropism,  the 
direction  and  movement  of 
the  cilia  may  be  modified  by 
electric  currents. 

Many  organisms  are  pro- 
vided with  cilia  about  the  oral 
opening,  vibration  of  which 
causes  currents  of  water  to 
flow  toward  the  mouth  so 
that  objects  adapted  for  food 
are  readily  caught.  Cilia  of 
this  kind  are  usually  longer 
than  those  used  for  locomo- 
tion. Beautiful  examples  are 
found  in  Rotifers. 

Cilia  also  occur  upon  the  cells  of  higher  animals  where 
they  subserve  different  purposes  in  the  economy  of  the 
animal  without  being  of  particular  benefit  to  the  cells 
themselves.  Thus  the  ciliated  cells  of  the  gills  of  Lamel- 
libranchiata  bring  currents  of  fresh  sea-water  to  the 
gills  of  the  animals  and  so  facilitate  the  aeration  of  the 
blood. 

The  respiratory  passages  of  vertebrates  are  lined  with 
ciliated  epithelium  whose  rhythmical  vibrations  assist  in 


FIG.  23. — Stylonychia  mytilus: 
wz.  Cilia  about  the  mouth  open- 
ing; c,  contractile  vacuole;  n, 
nucleus;  n',  para-nucleus;  a,  anus. 
(Clau's  Zoology.) 


78 


BIOLOGY:  GENERAL  AND  MEDICAL 


FIG.  25. — Diagram  of  a  sagittal  section 
of  a  Rotifer,  b,  Brain;  bl,  excretory  blad- 
der;  c,  cloaca,  the  common  opening  of 
digestive  and  reproductive  organs;  co, 
PIG.  24.— Spiral  path  of  Paramo?-  caelom;  e,  eyespot;  ex,  excretory  canal;  /, 
oium.  The  figures  1,  2,  3,  4,  etc.,  flame  cells ;f.g,  foot  gland;  ft,  foot;  g,  gut; 
show  the  successive  positions  oc-  m,  mouth;  m.f,  longitudinal  muscle  fibres; 
cupied.  The  dotted  areas  with  small  mx,  mastax;  o,  ovary;  ph,  pharynx,  s.g, 
arrows  show  the  currents  of  water  salivary  gland;  t,  tentacle;  tr,  trochus, 
draws  from  in  front.  (Jennings.)  or  cilia-bearing  disc.  (Galloway.) 


THE   MANIFESTATIONS    OF   LIFE  79 

removing  minute  inhaled  particles  from  the  deeper 
portions  of  the  respiratory  tract. 

The  Fallopian  tubes  are  also  lined  with  cilia  whose 
lashings  directed  toward  the  uterus  facilitate  the  passage 
of  the  mature  ovum  from  the  ovary  to  that  viscus. 

Flagella  are  longer  coarser  processes  not  clearly  differ- 
entiable  from  cilia,  but  usually  occurring  in  smaller 
numbers.  As  has  been  said,  custom  sanctions  the  use  of 
the  word  flagella  in  regard  to  the  cilia  of  bacteria.  In 


FIG.  26. — Trypanosoma   lewisi.     A  flagellate  parasite  of  the  blood  of  the  rat. 
X  1000. 

these  lowly  vegetable  organisms  they  may  appear  like 
cilia  in  that  they  sometimes  arise  from  all  parts  of  the 
surface  of  the  organism  or  like  the  flagella  of  the  protozoa 
in  that  they  arise  from  one  or  both  ends  of  the  cell. 

In  the  flagellata  they  may  also  arise  from  one  or  both 
ends,  and  may  be  single  or  multiple.  In  the  highly 
motile  forms  of  flagellate  protozoa,  such  as  the  trypano- 
somes,  they  usually  project  anteriorly  and  possess  a 
spiral  movement  by  which  the  animal  is  drawn  through 
the  fluid  medium  in  which  it  lives. 


80  BIOLOGY:  GENERAL  AND  MEDICAL 

There  is  but  one  flagellated  cell  in  the  higher  animals, 
the  spermatozoon,  in  which  the  single  flagellum,  called 
the  tail,  propels  the  cell  like  the  tail  of  a  tadpole  and 
follows  the  body  or  head.  The  purpose  of  the  flagellum 
is  to  enable  the  spermatozoon  to  ascend  the  reproduc- 
tive passages  (Fallopian  tubes)  until  it  meets  the  ovum 
for  fertilization. 

Contractile  Movements. — Certain  cells  undergo  a  physi- 
ological specialization,  by  which  they  are  enabled  to  con- 
tract and  so  produce  movements  advantageous  to  the  cell. 
It  is  difficult  to  determine  exactly  where  this  function 
begins.  It  might  be  attributed  to  the  amoeba  and 
explain  the  withdrawal  of  its  pseudopodia  were  it  not 
impossible  to  find  any  evidence  that  these  processes  in 
any  manner  differ  from  the  remainder  of  the  cytoplasm. 

In  vorticella  we  see  a  pedicle  or  stalk  consisting  of  an 
extension  of  the  cytoplasm  (ectoplasm)  endowed  with 
active  contractile  powers,  and  showing  a  peculiar  spiral 
structure  to  account  for  it. 

In  many  hydras  the  tentacles  are  armed  with  nettle- 
cells  or  stinging  cells,  irritation  of  which  causes  the 
sudden  projection  of  fine  stinging  cells  by  which  the 
prey  of  the  animal  is  paralyzed.  When  the  stinging 
function  is  performed,  the  hair-like  process  is  again 
withdrawn  through  a  contractile  specialization  of  that 
part  of  the  cell. 

As  the  scale  of  animal  complexity  is  ascended  and 
increase  of  size  and  differentiation  of  structure  becomes 
more  and  more  marked,  the  necessity  for  special  mechan- 
isms by  which  the  necessary  movements  for  carrying 
on  the  functions  is  more  imperatively  experienced, 
special  cells  and  groups  of  cells  become  endowed  with  a 
structure  adapting  them  for  contraction.  These  are 
known  as  muscle  cells  and  eventually  lose  most  of  their 
cellular  characteristics  through  the  differentiation  of  their 
substance  into  fibrillse  composed  of  alternating  discs, 
chemico-physical  disturbances  of  which  result  in  length- 
ening or  shortening  and  thus  in  movement. 


THE    MANIFESTATIONS    OF   LIFE 


81 


So  complex  does  this  structure  become  in  the  higher 
animals  that  the  exact  nature  of  the  contractile  phe- 
nomena has  not  yet  been  explained. 

METABOLISM. 

The  early  students  of  biology  believed  that  the  vital 
manifestations  .  depended  upon  a  special  force — vital 
force — peculiar  to  living  substance.  A  few  still  adhere 


FIG.  27. — Myograph,  an  instrument  used  for  recording  muscular  contractions. 
A  frog's  muscle  a,  >s  fixed  by  one  end  in  the  holder  b,  while  a  thread  c,  is 
fastened  to  the  other  tendon,  connecting  it  with  the  weight,  d.  When  the 
electrode  e,  stimulates  the  muscle,  the  movement  is  recorded  upon  the  revolving 
drum,  /.  (Feruwn.) 

to  this  belief  and  are  called,  in  consequence,  vitalists,  but 
an  ever-increasing  majority  see  nothing  in  the  life  proc- 
esses that  may  not  be  explained  by  the  laws  of  chemistry 
and  physics  and  are  therefore  known  as  chemico-physicists. 
When  the  activities  of  living  substance  are  carefully 
studied,  it  is  easy  to  determine  that  every  activity,  being 
an  expenditure  of  energy,  is  attended  with  molecular 
(chemical)  changes  in  its  composition,  usually  in  the 
form  of  oxidation  and  analogous  to  the  process  of  com- 


82  BIOLOGY:  GENERAL  AND  MEDICAL 

bustion.  We  are,  therefore,  led  to  the  conclusion  that 
the  chemical  activities  are  the  source  of  the  energy,  and 
that  all  vital  manifestations  are  physico-chemical  in 
nature. 

The  phenomenal  differences  between  vital  and  non- 
vital  substance  is  found  in  the  self-constructive  and  self- 
sustaining  character  of  the  former. 

As  has  been  shown  in  a  former  chapter,  there  is  no 
evidence  that  living  substance  is  at  present  self-existent. 
All  known  forms  spring  from  antecedent  forms  of  like 
kind  whose  origin  is  as  unknown  as  the  origin  of  matter 
and  force.  But  such  forms  of  living  matter  as  are  known 
begin  life  in  a  very  humble  form — mere  specks  of  proto- 
plasm constituting  the  spores,  seeds,  or  eggs  of  their 
parents — though  endowed  with  the  phenomenal  self- 
sustaining  and  self-constructive  powers. 

Remembering  that  every  activity,  being  an  expendi- 
ture of  energy,  results  in  oxidation,  and  this  in  alter- 
ations in  molecular  composition,  we  wonder  to  see  no 
visible  result  ensue.  The  secret  of  this  lies  in  the  wonder- 
ful power  of  adjustment  by  which  the  molecular  disturb- 
ances are  immediately  compensated  for  by  internal 
rearrangements  of  the  molecules. 

If  the  activities  be  increased  by  stimulation,  we  find 
that  the  compensatory  readjustments  are  of  limited 
extent  and  duration,  and  the  organism  soon  ceases  to  re- 
spond. It  is  fatigued,  or  it  rests,  or  it  enters  a  state  of 
vital  rigidity.  If  left  to  itself  for  a  time,  the  needed 
adjustments  take  place  and  activities  begin  again,  show- 
ing that  the  organism  still  lives. 

Suppose,  however,  the  stimulation  be  of  such  nature 
as  to  compel  the  organism  to  activities  of  more  prolonged 
duration,  attended  with  greater  oxidation  and  greater 
molecular  disturbance,  and  the  point  is  eventually  reached 
when  the  possibility  of  internal  adjustment  ends  and  re- 
action to  the  stimulant  ceases  not  because  it  is  tempo- 
rarily embarrassed  by  the  necessity  for  adjustment,  but 
because  adjustment  has  become  impossible.  The  crea- 
ture then  dies. 


THE   MANIFESTATIONS    OF   LIFE  83 

When  we  inquire  into  the  meaning  of  the  continuous 
activity  of  normal  life,  as  contrasted  with  the  temporarily 
suspended  activity  of  fatigue  and  the  permanently 
suspended  activity  of  death,  we  find  it  explainable 
through  a  study  of  the  nutritive  or  self-sustaining  power 
of  the  organism. 

Ordinary  activity,  exaggerated  activity,  exhausting 
and  fatal  activity,  being  followed  by  varying  degrees  of 
molecular  disturbance  through  combustion,  necessitate 
varying  degrees  of  molecular  reintegration;  i.e.,  the 
introduction  of  new  matter  to  replace  what  has  been  lost. 

Such  new  matter  constitutes  the  food  of  the  organism. 
A  food  may  therefore  be  defined  as  any  substance 
from  which  a  living  organism  is  able  to  derive  material 
for  its  sustenance  or  increase. 

It  is  a  common  observation  that  organisms  beginning 
their  life  histories  as  microscopic  masses  of  protoplasm 
eventuate  in  enormous  numbers  of  simple  or  in  enormous 
masses  of  complex  kind.  Increase  in  size  is  known  as 
growth;  increase  in  number,  as  reproduction.  Such  increase 
of  numbers  or  size  being  possible  only  through  increase  in 
the  actual  quantity  of  the  living  substance,  it  becomes  clear 
that  that  substance  is  endowed  with  the  capacity  of  form- 
ing more  substance  of  its  own  kind.  Metabolic  activity 
manifested  by  increase  of  the  living  substance  is  anabolic, 
and  takes  place  by  chemical  synthesis  of  molecular  groups 
to  higher  and  higher  compounds  until  protoplasm  is 
reached.  Metabolic  activity  manifested  by  the  analysis, 
by  oxidation,  of  the  already  formed  protoplasm,  is  said  to 
be  katabolic,  and  results  in  the  liberation  of  the  potential 
energy  in  the  form  of  force  and  heat.  In  plants  anabolism 
preponderates;  in  animals  katabolism  preponderates. 
The  explanation  is  found  in  the  inactivity  of  plants  as 
compared  with  animals. 

Living  substance  or  protoplasm  is  the  most  complexly 
compounded  of  all  the  substances  known  to  the  chemist. 
Indeed  it  is  so  complex  in  chemical  structure  that  its 
exact  composition  is  unknown  and  no  correct  formula 
for  it  has  been  worked  out. 


84  BIOLOGY:  GENERAL  AND  MEDICAL 

Protoplasm  stands  at  the  head  of  a  list  of  compounds 
known  as  proteins,  some  of  which  are  well-known,  yet 
not  one  of  which  is  correctly  known  as  regards  its  true 
chemical  composition.  The  reason  becomes  obvious 
when  a  few  of  the  formulae  suggested  are  examined. 
Thus  one  chemist  who  studied  the  hemoglobin  from 
the  dogs'  blood  finds  it  represented  by  the  formula 
C726H1171N194O214S3.  One  of  the  various  formulas  sug- 
gested for  egg-albumen  is  C204H322N52O66S2.  The  com- 
plexity is  so  great  that  no  two  chemists  arrive  at  the 
same  result.  If  such  is  the  complexity  of  fixed  and 
relatively  stable  egg-albumen,  how  much  more  elaborate 
the  structure  of  the  subtle  and  evanescent  living  proto- 
plasm must  be  we  can  scarcely  conjecture. 

Gustav  Mann,  however,  very  properly  points  out 
that  we  may  fall  into  error  in  this  particular.  He  says: 
"To  many  people  a  living  cell  consists  of  'protoplasm,' 
a  substance  they  imagine  to  be  one  exceedingly  complex 
body.  They  do  not  realize  that  in  a  cell  we  have  a  not 
very  large  number  of  comparatively  simple  compounds 
which  only  collectively  form  the  protoplasm.  What 
constitutes  life  is  the  presence  of  a  number  of  such 
'organic '  compounds  capable  of  mutually  reacting  upon 
one  another,'  and  thereby  giving  rise  to  new  compounds, 
which  cannot  react  chemically  with  the  mother  sub- 
stance from  which  they  are  derived,  but  which  by 
interacting  with  new  radicals  give  rise  to  a  cycle  of 
events." 

To  learn  the  process  by  which  protoplasm  is  built  up, 
one  might  imagine  that  the  simplest  method  would  be  to 
follow  the  successive  steps  in  its  disintegration,  learn  the 
products  of  its  analysis,  and  then,  by  retracing  the  steps 
from  the  simple  to  the  complex,  arrive  at  an  approxi- 
mately accurate  result. 

It  is  true  that  when  protoplasm  dies  it  undergoes  a 
speedy  dissolution,  terminating  in  a  number  of  well- 
known  simple  compounds,  but  though  the  stages  in  the 
disintegration  process  seem  to  be  so  brief,  the  successive 


THE   MANIFESTATIONS    OF    LIFE  85 

steps  pass  through  an  enormous  series  of  transformation 
products  so  rapidly,  that  they  cannot  be  followed. 

Indeed,  if  we  start  with  a  relatively  stable  protein,  like 
egg-albumen,  and  endeavor  to  resolve  it  into  less  and 
less  complex  compounds,  we  still  find  ourselves  working 
with  a  complexity  yielding  many  series  of  compounds 
until  we  are  lost  in  an  impenetrable  physiologico-chemi- 
cal  labyrinth,  beset  on  every  side  with  a  polysyllabic 
nomenclature  that  only  increases  our  bewilderment. 

We  cannot,  therefore,  in  the  present  state  of  knowl- 
edge pretend  to  follow  the  elaborate  synthesis  of  living 
matter.  We  can,  however,  analyze  that  matter  and 
discover  the  elementary  substances  of  which  it  is  com- 
posed, and  this  has  been  done  again  and  again  with 
interesting  and  important  results. 

Thus  a  chemical  analysis  of  cells  shows  them,  without 
exception,  to  be  composed  of  C,  O,  H,  N,  S,  P,  K,  Ca,  Mg, 
and  Fe. 

It  must  not  be  supposed  that  the  elements  mentioned 
comprise  the  full  list  of  all  that  may  be  found  in  either 
vegetable  or  animal  tissues;  instead  they  form  the  indis- 
pensable list  to  which  others  may  be  or  commonly  are 
added  with  advantageous  results  to  the  particular  organ- 
isms. Thus,  for  example,  Si  is  not  enumerated,  yet  it  is 
frequently  present  in  plants  and  of  benefit  by  increasing 
the  rigidity  of  the  tissues. 

If  living  substances,  either  animal  or  vegetable,  re- 
quire at  least  ten  elementary  substances  for  the  elabora- 
tion of  their  tissues,  it  is  evident  that  they  cannot  per- 
form their  vital  functions  when  the  supply  of  these 
elements  fails. 

It  is  also  evident  that  no  substance  can  be  so  well 
adapted  for  the  supply  of  the  essential  elements,  in  the 
most  useful  combinations,  as  living  substance  itself,  which 
explains  why  living  beings  of  the  highest  kind  so  uni- 
versally live  upon  living  things  of  lower  kinds.  The 
next  most  useful  material  would  be  that  which  had 
been  living,  but  is  in  process  of  dissolution  into  similar 
compounds  of  still  assimilable  quality. 


86  BIOLOGY:  GENERAL  AND  MEDICAL 

It  is,  however,  evident  that  primordial  forms  of  life 
could  neither  feed  upon  antecedent  life  nor  its  derivatives, 
hence  we  find  at  the  bottom  of  the  scale  of  living  things, 
forms  that  are  able  to  seize  upon  relatively  simple  in- 
organic compounds  combining  them  into  more  and  more 
complex  compounds  and  integrating  them  into  living 
protoplasm,  while  at  the  top  of  the  scale  of  life  we  find 
organisms  whose  appearance  must  have  come  relatively 
late  in  time,  that  are  unable  to  make  use  of  any  of  the 
simple  inorganic  substances,  and  are  absolutely  depend- 
ent upon  the  lower  antecedent  forms  for  food. 

How  the  animal  substance  reintegrates  itself  and 
builds  up  additional  animal  substance  by  appropriating 
to  its  uses  the  equally  complex  substance  of  other 
individuals  or  the  slightly  less  complex  products  of  their 
disintegration,  it  is  beyond  the  knowledge  of  chemists 
to  give  any  adequate  information,  for  we  neither  know 
the  nature  of  the  substance  being  integrated  nor  that  of 
the  substances  by  which  it  is  being  integrated. 

On  the  other  hand,  when  we  inquire  how  the  more 
simple  vegetable  organisms  are  nourished,  the  problem 
is  simplified,  for,  though  the  vegetable  protoplasm  is 
still  too  complex  for  us  to  follow  its  transformations, 
the  compounds  with  which  and  upon  which  it  works  are 
so  simple  and  so  well-known  that  we  can  easily  follow 
them  through  many  transformations.  In  doing  this, 
however,  we  find  the  living  substance  capable  of  perform- 
ing miracles  of  chemical  synthesis,  the  experimental 
reproduction  of  which  is  impossible. 

Of  the  numerous  elements  that  are  said  to  enter  into 
the  composition  of  living  substance  many  have  been 
mentioned  that  find  no  place  in  the  hypothetical  struc- 
ture of  the  proteid  molecule.  It  is  by  no  means  certain 
that  these  elements  find  a  place  in  the  composition 
of  protoplasm.  They  are  discovered  by  an  examina- 
tion of  masses  of  tissue  composed  of  protoplasm  and  its 
numerous  products,  not  by  examination  of  the  elemen- 
tary substance  itself.  A  single  elementary  mass  of  pro- 
toplasm— a  cell — is  so  small  and  the  quantity  of  these 


THE    MANIFESTATIONS    OF   LIFE  87 

elements  so  minute  that  they  must  inevitably  elude 
detection.  It  is  not  known,  therefore,  whether  they 
are  present  or  not.  It  is,  however,  known  that  it  is 
only  in  the  presence  of  these  elements  that  the  functions 
of  life  can  be  carried  on.  In  the  vegetable  world  this  is 
so  fundamental  in  importance  that  if  a  single  one  of 
these  elements  —  S,  P,  K,  Ca,  Mg,  and  Fe  —  is  absent, 
normal  development  is  impossible. 

It  is  supposed  that  the  living  matter  —  protoplasm  — 
is  purely  protein  in  character,  and  consists,  like  egg- 
albumin,  hemoglobin  and  other  proteins,  of  some  com- 
bination of  C,  H,  0,  N,  and  S,  and  that  the  additional 
elements  serve  as  electrolytes  by  which  its  functions 
are  carried  on. 

When  we  come  to  study  the  materials,  out  of  which 
protoplasm  can  be  built  up,  most  interesting  experi- 
mental studies  in  plant  life  are  available. 

Thus  Proskauer  and  Beck  found  that  the  tubercle 
bacillus,  one  of  the  bacteria,  or  lowest  forms  of  vegetable 
life,  can  grow  well  in  a  mixture  consisting  of: 

Commercial  ammonium  carbonate  (NH4H  CO3)  .        0.35 
Primary  potassium  phosphate  (K-jPOg)  ........       0.15 

Magnesium  sulphate  (MgSO4)  ................       0.  25 

Glycerin  (C3H8O3)     .........................        1.5 

Water  (H20)  .............................  ad  100. 

Raulin  made  a  most  painstaking  study  of  the  nutrition 
of  a  mould,  Aspergillus  niger,  testing  it  in  every  possible 
way  and  finally  discovering  that  its  maximum  growth 
took  place  in  a  solution  containing: 

Water    .....................  1500  .  00  grams 

Cane-sugar  ..................  70  .  00  grams 

Tartaric  acid    ...............  4  .  00  grams 

(NH4),PO4      ................  0.60  grams 

K,CO3   .....................  0.60  grams 


MgCO3 

(NH4)2SO4 

ZnSO4 


0.40  grams 
0.25  grams 
0  .  07  grams 
FeSO4    .....................         0  .  07  grams 

K2SiO3    ....................          0.07  grams 


So  precise  was  this  work  of  Raulin,  that  he  found  the 
least  variation  in  any  of  these  ingredients  produced  a 


88  BIOLOGY:  GENERAL  AND  MEDICAL 

quantitative  difference  in  the  weight  of  the  dried 
residuum  of  mould  secured. 

If  we  wish  to  study  plant  nutrition  and  discover  out 
of  what  materials  the  substance  of  the  growing  organism 
is  elaborated,  it  can  be  done  with  exactness  by  means 
of  a  water  culture.  This  consists  of  distilled  water  to 
which  are  added  known  quantities  of  such  compounds 
as  are  essential  to  the  life  processes  of  the  plant,  v.  d. 
Crone  recommends  the  following  solution  which  is  one 
of  the  best: 

Distilled    water   1000-2000  c.c. 

Potassium  nitrate    1  gram 

Ferrous  phosphate    0.5  gram 

Calcium  sulphate 0.25  gram 

Magnesium  sulphate 0 . 25  gram 

An  examination  of  these  ingredients  will  discover  the  S 
in  two,  P  in  one,  K  in  one,  Ca  in  one,  Mg  in  one,  and  Fe 
in  one,  thus  this  solution  furnishing  all  of  the  electrolytes 
essential  to  plant  life,  a  seed  or  spore  moistened  with 
it  is  able  to  set  in  motion  those  chemical  processes  by 
which  its  integration  is  effected.  Some  of  the  essential 
elements  of  the  protein  of  the  protoplasm  are,  however, 
conspicuous  by  their  absence,  Where,  for  example, 
are  the  O,  H,  and  C?  There  are  two  sources  from  which 
these  may  be  obtained,  O  from  the  atmosphere  into 
which  the  plant  grows,  H  from  the  water  into  which  its 
roots  extend,  and  in  each  of  which  more  or  less  CO2  is 
dissolved  and  may  yield  the  C. 

Absorbing  the  H2O,  the  CO2,  and  the  0,  the  germ  of 
living  substance,  by  virtue  of  the  powers  it  already 
possesses,  with  the  aid  of  the  electrolytes  with  which  it  is 
supplied  in  the  solution,  begins  operations  by  combining 
these  molecules  to  form  more  complex  compounds,  some 
of  which  it  no  doubt  immediately  carries  further  through 
intermediate  steps  to  the  actual  plant  protein,  some  of 
which  it  stores  up  for  time  of  future  need,  and  some  of 
which  it  uses  for  the  scaffolding  in  which  its  increasing 
active  substance  is  to  be  supported  and  by  which  it  is  to 


THE    MANIFESTATIONS    OF   LIFE 


89 


be  protected.  It  is  possible  to  follow  what  are  presum- 
ably the  successive  steps  in  the  formation  of  one  of  the 
best-known  of  the  vege- 
table products,  starch. 
Thus,  in  conditions  such  as 
prevail  in  the  water  cul- 
ture, to  which  reference  has 
been  made,  we  have  found 
available  for  use  O  in  the 
air,  water  (H20)  in  which 
the  salts  required  are  dis- 
solved, and  CO2  in  both  the 
air  and  water,  in  small 
quantities.  We  have, 
therefore,  0,  H2O,  and  CO2 
to  work  with.  The  first 
step  in  the  process  seems 
to  be  the  extraction  of  the 
C  atom  from  the  CO2  and 
its  addition  to  the  H2O 
molecule  thus: 

C02  +  H2O  =  CH20  (formic 
aldehyde)  +  02. 

If  we  study  the  metabolism 
of  growing  plants — photo- 
synthesis— we  find  that 
this  actually  takes  place,  for 
in  the  gases  given  off  there 
is  an  increase  in  the  O  which 
can  only  be  accounted  for 
by  the  abstraction  of  the  C 
from  the  CO2. 

As    the   Synthetic    process 

is  continued  we  find: 

6CH2O  =  C6H1206  (monosaccharide) 
formed  by  a  rearrangement  of  atoms  to  make  a  more 


FIG.  28. — Water  cultures  of  Fagopy- 
rum  esculentum.  I.  In  nutrient  solution 
containing  potassium;  II.  in  nutrient 
solution  without  potassium.  Planta 
reduced  to  same  scale.  (After  Nobbe.) 


90  BIOLOGY:  GENERAL  AND  MEDICAL 

complex  molecule,  and  then  a  still  more  complex  arrange- 
ment: 

nC6H12O6  -  nH2O  =  (C6H10O5)n  (starch  and  cellulose) 
by  which  starch  and  cellulose  are  made. 

Photosynthesis,  or  the  synthetic  process  described,  is 
peculiar  to  plant  life,  and  to  those  forms  of  plants  that 
contain  chlorophyl.  It  is  also  only  possible  in  direct  or 
diffused  sunlight.  It  is  the  basis  of  holophytic  nutrition — 
i.e.,  nutrition  of  the  purely  plant  type. 

At  this  point  we  reach  products  capable  of  subserving 
many  different  purposes.  Doubtless  further  more  com- 
plex syntheses,  as  of  proteins,  are  immediately  effected, 
though  the  cellulose  being  a  structural  element  of  import- 
ance to  the  plants  is  commonly  deposited  in  permanent 
form  where  needed,  and  the  starch  may  be  temporarily 
deposited  in  scattered  granules  or  in  dense  aggregations, 
as  in  potatoes,  peas,  beans,  fruits,  etc.,  until  needed  for 
further  transformations. 

When  the  starch  is  to  be  further  utilized  in  the  nutri- 
tion of  the  plant,  it  is  converted  to  sugar  and  distributed 
by  the  sap  in  which  the  sugar  is  dissolved,  this  distribu- 
tion being  known  as  metastasis. 

At  this  stage  we  also  reach  the  point  at  which  the  utili- 
zation of  the  plant  products  by  animals  becomes  possible, 
starch  forming  one  of  the  most  valuable  foods  of  the 
higher  animals.  The  nutrition  of  animal  organisms 
consists  in  the  analysis  and  resynthesis  of  the  vegetable 
products — fats,  starch,  and  protein — and  is  described  as 
holozoic.  It  includes  no  photosynthesis  and  no  ability 
to  utilize  H2O,  CO2,  and  O  in.  the  synthesis  of  starches  and 
sugars. 

Beyond  this  point  the  progress  of  the  synthetic  process 
can  no  longer  be  followed,  because  the  complexity  of  the 
molecular  compounds  becomes  too  great. 

Thus  far  foods  have  been  discussed  only  with  reference 
to  growth,  though  growth  or  anabolism  is  an  active  process 
in  which  energy  is  expended.  In  the  activities  of  animal 
life  enormous  expenditures  of  energy  have  constantly  to 


THE   MANIFESTATIONS   OF   LIFE  91 

be  met.  It  is  not  enough,  for  example,  that  an  animal 
build  up  great  quantities  of  muscular  tissue  to  meet  the 
requirements  of  locomotion;  it  is  quite  as  necessary  that 
it  provide  for  neutralizing  the  loss  of  energy  resulting 
from  these  muscles  in  action.  If  this  were  not  done  the 
muscles  would  waste  through  self -combustion  as  they  were 
called  into  action,  and  would  soon  disappear.  The  com- 
pensation is  afforded  by  the  food.  An  equivalent  of  the 
expenditure  in  the  form  of  assimilable  material  is  sup- 
plied to  the  muscle-cells  as  their  food,  and  the  katabolic 
destruction  of  the  muscle-tissue  itself  thus  prevented. 
So  the  energy  stored  in  the  food  is  utilized  by  the  living 
organism  and  its  own  structure  saved  from  destruction. 

So  "food"  furnishes  the  means  of  growth  through 
anabolism  or  further  synthesis,  and  the  source  of  heat  and 
energy  through  katabolism  or  analysis. 

REPRODUCTION. 

As  living  beings  are  subject  to  such  wear  and  tear  as 
results  from  their  activities  and  to  accidents  of  various 
kinds,  the  persistence  of  life  is  dependent  upon  the 
ability  of  living  substance  to  reproduce  its  own  kind, 
Reproduction  is,  therefore,  a  fundamental  manifestation 
of  life. 

The  new  individual,  no  matter  how  produced,  is 
endowed  with  potentialities  and  possibilities  the  sum  of 
which  constitute  youth  and,  therefore,  constitutes  a  new 
generation  qualified  to  repeat  not  only  the  structure,  but 
also  the  functions  of  its  parent. 

As  will  be  shown  in  a  future  chapter,  the  particular 
form  of  reproduction  varies  with  the  simplicity  or  com- 
plexity of  structure,  yet  all  forms  of  reproduction  are 
ultimately  referable  to  simple  phenomena,  such  as  are 
seen  in  the  most  simple  forms  of  life — i.e.,  the  cells — and 
consist  of  cell-division.  This  is  a  manifestation  the 
cause  of  which  has  long  been  sought  by  biologists  with 
little  success.  It  was  at  one  time  supposed  to  depend 


92  BIOLOGY:  GENERAL  AND  MEDICAL 

upon  some  simple  physical  condition  and  even  attributed 
to  disproportions  between  the  absorbing  surface  through 
which  the  cell  was  nourished  and  the  amount  of  cell 
contents  to  be  nourished. 

Thus,  as  a  cell  grows  the  contents  increase  in  three 
dimensions  while  the  surface  increases  in  but  two.  It 
was  supposed  that  when  a  certain  cubical  content  was 
reached,  the  cell  became  embarrassed  by  its  inability  to 
secure  nourishment  and  so  was  compelled  to  undergo 
some  compensatory  change,  the  most  frequent  being 
division.  This  explanation  is,  however,  inadequate,  for 
some  cells  are  naturally  and  uniformly  larger  than  others, 
and  among  cells  whose  nourishment  is  regularly  effected 
through  the  intracellular  digestion  of  ingested  living  or- 
ganisms— holophagous — or  of  food  particles,  it  could  not 
apply. 

It  would  seem,  therefore,  to  be  a  hereditary  peculiarity 
of  the  cell  and  to  be  determined  by  conditions  intrinsic 
in  the  cell  itself. 

This  inability  to  fully  comprehend  the  conditions 
leading  to  multiplication  or  reproduction  forbids  an 
intelligent  understanding  of  the  process  and  limits  us 
to  the  description  of  what  is  to  be  seen  without  enabling 
us  to  explain  it. 

REFERENCES. 

O.  HERTWIG:     "Die  Zelle  und  die  Gewebe,"  Jena,  1892,  1905 
MAX  VERWORN:     "Allgemeine  Physiologic,"  Jena,  1909. 
H.  S.  JENNINGS:  "  Behavior  of  the  Lower  Organisms,"  N.  Y.,  1906. 
JACQUES  LOEB:     "The  Dynamics  of  Living  Matter/'  N.  Y.,  1906. 
E.  STRASBURGER,  H.  SCHENCK,  F.  NOLL,  AND  G.  KA"RSTEN.     "A 

Text-book  of  Botany,"  Third  English  Edition  by  W.  H. 

Lang,  N.  Y.  and  London,  1908. 


CHAPTER  V. . 
THE  CELL. 

The  most  simple  living  beings  consist  of  single  struc- 
tures, i.e.,  cells,  and  are  known  as  protozoa  when  of 
animal  nature,  protophyta,  when  of  vegetable  nature. 
More  complex  beings  are  composed  of-  an  increasing 
number  of  cells  which  eventually  become  innumerable. 
Such  are  known  as  metazoa  and  metaphyta,  respectively. 
An  analysis  of  structure  thus  leads  to  the  cell  as  the 
unit,  and  before  complexly  organized  beings  can  be 
understood  a  knowledge  of  cells  becomes  imperative. 

Until  the  nineteenth  century,  microscopy  had  not 
reached  a  point  at  which  it  was  able  to  place  satisfactory 
interpretation  upon  the  minute  structure  of  either 
plants  or  animals.  This  came  in  1838  when  Schleiden, 
a  German  botanist,  showed  that  vegetable  tissues  were 
composed  of  various  combinations  of  more  or  less  similar 
living  units  formed  in  a  true  and  orderly  manner,  and 
Schwann  discovered  the  same  to  be  true  for  animals. 

The  living — vital — nature  of  the  structural  units 
formed  the  basis  of  Kolliker's  new  science  of  histology, 
became  the  foundation  of  a  new  conception  of  physiology 
in  the  hands  of  Verworn,  and  was  made  the  basis  of 
modern  pathology  by  Virchow. 

The  term  cell  came  to  be  applied  to  the  elementary 
structural  units  in  a  peculiar  way.  In  1665  Robert 
Hooker,  examining  a  piece  of  cork  with  the  microscope, 
found  it  made  up  of  a  large  number  of  minute  compart- 
ments, which  reminded  him  of  the  cells  of  a  monastery. 
He,  therefore,  called  each  "a  cell,"  and  though  time  has 
revealed  the  true  nature  of  the  elements,  and  showed  them 
to  be  other  than  closed  spaces,  the  name  has  survived. 
93 


94  BIOLOGY:  GENERAL  AND  MEDICAL 

The  cell  was  originally  conceived  to  be  an  anatomical 
unit,  and  it  was  supposed  that  all  cells  presented  more 
or  less  uniformity  of  structure;  but  it  is  now  known  that 
their  structures  may  be  quite  dissimilar.  Indeed,  such 
latitude  now  enters  into  the  concept  "cell"  that  it  becomes 
almost  impossible  to  define  it.  As  here  used,  the  term 
cell  signifies  a  structure  capable  of  displaying  all  of  the 
vital  manifestations,  but  not  capable  of  resolution  into 
simpler  vital  structures. 

The  cells,  thus  understood,  vary  widely  among  them- 
selves as  to  size,  morphology,  independence,  and  function. 

In  regard  to  size,  there  is  an  enormous  difference. 
Some  cells,  as  the  bacteria,  are  so  minute  as  to  be  visible 
only  to  the  highest  powers  of  the  microscope,  and  there 
is  every  indication  that  there  are  cells  so  minute  that  no 
microscope  thus  far  invented  can  define  them.  Other 
cells,  as  certain  eggs,  are  large  enough  to  be  visible  to  the 
naked  eye  without  difficulty.  Size  is,  therefore,  an  un- 
important quality  of  the  cell. 

In  morphology  the  cells  differ  almost  as  widely  as  in 
size.  Certain  of  the  most  lowly  forms  of  life  consist  of 
microscopic  specks  of  protoplasmic  jelly  in  which  no 
definite  structure  can  be  made  out,  so  that  it  has  long 
been  controversial  whether  they  are  provided  with  even 
so  important  an  organ  as  a  nucleus.  Other  cells,  such 
as  the  mammalian  ovum  or  egg,  are  complexly  con- 
structed and  provided  with  many  well-known  and  clearly 
defined  parts. 

Taking  the  mammalian  ovum  as  a  good  type  for  study, 
we  find  it  conforming  to  the  primitive  conception  from 
which  the  term  "  cell"  was  derived;  i.e.,  it  is  a  globular  or 
spherical  mass  of  living  substance,  shut  in  by  a  definite 
envelope,  membrane,  or  "wall."  This  primitive  idea 
of  all  cells  being  definitely  inclosed  by  a  "wall"  has  long 
been  abandoned,  for  it  is  now  well  known  that  there  are 
many  more  cells  without  than  with  such  a  structure. 
What  is  true  of  the  cell  wall  or  membrane  is  true  of  all 
the  cell  structures  except  the  protoplasm  and  the  nucleus. 


THE   CELL  95 

There  are  no  cells  without  protoplasm,  and  it  is  con- 
troversial whether  there  are  cells  without  a  nucleus. 
There  are,  however,  cells  in  which  no  nuclei  have  yet 
been  seen. 

Taking  it  for  granted  that  our  inability  to  recognize 
a  nucleus  in  certain  cells  is  evidence  that  none  exists,  we 
find  the  most  primitive  form  of  cell  to  be  a  minute 
undifferentiated  speck  of  protoplasmic  jelly.  This  shows 
us  that  the  protoplasm  or  cytoplasm,  as  it  is  now  usually 
called,  is  the  essential  living  substance.  Many  attempts 
have  been  made  to  show  that  the  cytoplasm  is  of  definite 
texture — thus,  Flemming  looked  upon  it  as  filamentous; 
Biitschli,  as  frothy  or  honeycombed,  and  Altmann,  as 
granular.  Its  true  nature  is  still  controversial.  It  is  usual 
to  describe  it  as  consisting  of  a  structureless  matrix  of  clear 
jelly,  the  hyaloplasm,  in  which  certain  granules,  the 
spongioplasm,  polioplasm,  or  mitochondria,  are  suspended. 

The  granules  are  of  various  quality,  as  is  shown  by 
their  diverse  micro-chemical  reactions.  This  is  well 
shown  by  an  examination  of  human  white  blood  cor- 
puscles treated  with  a  mixture  of  colors,  such  as  make  up 
Ehrlich's,  Biondi's,  Wright's,  Jenner's,  or  Romanowski's 
stains.  When  a  carefully  prepared  film  of  human  blood 
is  stained  with  one  of  these  reagents,  we  find  the  white 
corpuscles  showing  certain  variations  that  enable  them 
to  be  assigned  to  well-marked  classes.  Thus,  about  70 
per  cent,  show  a  cytoplasm  rich  in  granules  that  have 
assumed  a  purple  color,  and  are  known  as  neutrophilic 
granules;  about  25  per  cent,  are  without  granules  and 
are  called  hyaline  cells,  and  the  remainder  are  chiefly 
made  up  of  cells  filled  with  coarse  round  granules  that 
stain  a  brilliant  red  color — the  eosinophile  cells. 

It  may  naturally  be  inferred,  and  the  inference  is 
probably  correct,  that  these  diversely  reacting  granules 
are  different  in  nature  and  function,  though  it  has  not 
been  determined  what  the  office  of  the  granules  is. 
Altman,  who  first  described  cytoplasm  as  granular,  called 
.  the  granules  bioblasts  and  conceived  that  they  were  the 


96 


BIOLOGY:  GENERAL  AND  MEDICAL 


actual  source  of  the  cell  life;  but  his  view  has  been 
abandoned,  and  we  now  suppose  that  the  granules  of 
the  spongioplasm  are  composed  of  those  substances 
formed  by  the  living  substances  and  useful  in  its 
various  activities.  From  this  point  of  view  these 
granules  are  not  permanent  structures,  but  appear  and 
disappear  as  they  are  prepared  or  employed. 


Etofibsm  ____ 


Joreya  inciosutes 
FIG.  29.—  Diagram  of  a  cell.     (Huber.) 

It  is  further  found  difficult  to  say  what  granules  actu- 
ally belong  to  the  substance  of  the  cytoplasm  or  may  be 
temporarily  harbored  by  it.  Thus  in  many  cells  of  the 
higher  animals  we  find  granules  that  are  reserve  stuffs 
held  by  the  cells  until  needed  elsewhere.  In  the  cells 
of  the  resting  salivary  gland  large  numbers  of  granules 
are  found  which  are  absent  after  the  gland  has  been  for 
some  time  active.  These  granules,  used  up  during  the 
glandular  activity,  were  composed  of  the  antecedents 
of  substances  supplied  by  the  cells  to  the  glandular  secre- 


THE    CELL  97 

tion,  were  products  of  cellular  activity,  but  were  not 
components  of  the  essential  vital  structure  of  the  cyto- 
plasm. In  the  cells  of  the  liver  it  is  common  to  hnd 
globules  of  fat  and  glycogen  especially  after  a  meal  rich 
in  fats  and  carbohydrates,  but  being  temporarily  present 
they  cannot  be  essential  components  of  the  cytoplasm. 

In  the  larger  free  cells,  such  as  amoeba  and  paramcecium, 
we  find  granules  (physodes)  of  temporary  occurrence 
which  are  probably  the  not  yet  utilized  products  result- 
ing from  the  intracellular  digestion  of  the  smaller  entities 
upon  which  the  organisms  feed. 

In  vegetable  cells  there  is  even  less  difficulty  in  differ- 
entiating certain  large  coarse  granules  not  essential 
components  of  the  cytoplasm,  yet  contained  in  that  sub- 
stance and  destined  for  the  performance  of  definite 
functions.  These  are  the  chromatophores,  which  are 
the  antecedents  of  the  chloroplasts,  by  which  chlorophyl 
is  formed,  and  the  chromoplasts  by  which  are  elaborated 
the  pigments  by  which  the  flowers  and  fruits  are  colored. 
Plant  cells  also  contain  starch  and  aleurone  granules  in 
many  cases.  Many  plant  cells  are  also  distended  with 
sap  which  takes  the  form  of  vacuoles  that  soon  become 
too  large  to  be  mistaken  for  granules. 

It  is  thus  seen  that  the  cytoplasm  is  not  only  granular 
because  of  its  spongioplasm,  but  also  because  of  adventi- 
tious granules  constituting  the  deutoplasm  or  paraplasm, 
matters  temporarily  contained  in  it. 

It  must,  however,  occur  to  the  student  that  there  is 
no  criterion  for  the  separation  of  paraplasm  and  spongio- 
plasm other  than  our  ability  to  recognize  the  nature  and 
purpose  of  the  latter  and  our  inability  to  do  so  with 
regard  to  the  former. 

The  second  essential  structure  of  the  cell  is  the  nucleus. 
In  its  most  highly  developed  form  this  is  a  highly  com- 
plex organ.  It  appears  as  a  spheroidal  protoplasmic 
body  whose  size  bears  a  fairly  constant  relation  to  the 
size  of  the  cell  though  the  proportion  differs  greatly  in 
7 


98  BIOLOGY:  GENERAL  AND  MEDICAL 

cells  of  different  kinds.  It  is  usually  enclosed  in  a  hya- 
line membrane,  known  as  the  nuclear  membrane,  and 
consists  of  substance  divisible  into  a  part  that  is  struc- 
tureless and  probably  fluid,  known  as  the  nuclear  j-uice, 
or  karyoplasm,  and  material  that  is  for  the  most  part 
filamentous  and  known  as  the  nuclear  substance  or 
karyomitome.  Staining  reagents  show  that  the  struc- 
tures comprising  the  nucleus  are  chemically  different 
from  those  of  the  cytoplasm,  and  a  large  part  of  the 
science  of  histology  has  been  the  result  of  observations 
made  upon  cells  and  tissues  subjected  to  what  is  called 
nuclear  staining. 

The  microchemic  study  of  the  nuclear  structures  has 
not  been  very  useful,  except  that  it  has  shown  them  to  be 
dissimilar  in  composition.  The  nuclear  membrane  is  said 
to  consist  of  amphipyrenin,  the  karyoplasm  of  paralinin, 
and  the  karyomitome  of  two  substances,  one  of  which 
does  not  react  with  the  stains,  and  is  called  linin,  the 
other  with  a  strong  affinity  for  all  nuclear  stains,  being 
known  as  nuclein  or  chromatin.  In  some  nuclei  a  small 
distinct  body  or  nucleolus  occurs.  It  is  composed  of 
pyrenin. 

Of  these  various  structures  the  nuclein  or  chromatin 
is  of  paramount  importance,  being  composed  of  a  number 
of  units,  distinctly  visible  only  at  the  time  of  cell  multipli- 
cation and  known  as  chromosomes.  Much  will  be  said  of 
these  bodies  in  connection  with  karyokinesis  or  nuclear 
division  and  in  considering  the  problems  of  heredity. 

The  great  majority  of  cells  possess  a  single  nucleus; 
some  of  the  unicellular  organisms,  as  the  protozoa  are 
provided  with  two,  the  larger  and  more  distinct  being 
known  as  the  macronucleus,  the  smaller  and  less  con- 
spicuous as  the  micronucleus  or  paranucleus. 

Some  lowly  organisms  of  considerable  size  consist  of 
masses  of  undifferentiated  protoplasm  containing  many 
nuclei.  Such  are  known  as  plasmodia,  and  in  some  cases 
as  the  myxomycetes,  are  known  to  be  formed  by  the 
coalescence  of  many  cells. 


THE    CELL 


99 


The  cells  of  the  metazoa  and  metaphyta  rarely  contain 
more  than  one  nucleus,  the  striking  exception  being  the 
bone  corpuscles  or  myeloplaxes  found  in  the  red  marrow. 

The  shape  of  the  nucleus,  though  usually  spheroidal, 
may  vary.  In  elongate  cells  it  is  highly  prolate,  and  in 
cells  subject  to  much  compression  may  be  distinctly 
oblate.  In  old  cells  the  nuclei  may  be  irregular  in 


FIG.  30. — Cells  with  variously  shaped  nuclei,  a,  Vorticella,  a  ciliated 
infusorian  with  a  sausage-shaped  nucleus,  b,  Stentor,  a  ciliated  infusorian 
with  a  rosary-like  nucleus,  c,  Cells  from  the  silk  glands  of  a  caterpillar,  with 
nuclei  branched  like  stag's  antlers.  (After  Korschelt.) 


Rarely  in  the  cells  of  certain  arthropods  the 
nuclei  may  be  branched  or  even  reticular.  Diseased 
cells  may  also  show  breaking  up  or  fragmentation  of  the 
nucleus — karyorrhexis — or  solution  of  the  nuclear  mate- 
rials— karyolysis. 

The  cell  wall  that  formed  so  essential  a  part  of  the 
primitive  conception  of  the  cell  is  an  extremely  variable 
structure.  The  lowest  forms  of  life  are  commonly  with- 
out it,  and  the  greater  number  of  the  cells  of  the  metazoa 
are  without  it. 


100  BIOLOGY:  GENERAL  AND  MEDICAL 

In  its  most  primitive  form  one  sees  nothing  but  a 
transparent  hyaline  border  to  an  elsewhere  granular  cy- 
toplasm. Under  these  conditions  it  is  sometimes  spoken 
of  as  the  ectosarc,  to  differentiate  it  from  the  endosarc 
or  cell  substance.  As  the  evolution  of  the  structure  is 
followed,  it  is  found  that  it  modifies  certain  of  the  phys- 
iological manifestations  of  the  cell.  Thus,  the  amoeba 
that  has  no  true  cell  wall  is  actively  amoeboid  and 
appears  able  to  take  in  food  particles  through  any  part 
of  its  surface.  In  more  highly  specialized  protozoa 
(corticata),  such  as  Paramcecinum,  Kerona,  and  Vorti- 
cella,  there  is  a  well-defined  somewhat  rigid  cell  envelope 
which  prevents  amoeboid  movement  and  determines 
that  food  can  be  ingested  only  at  a  given  point  in  the 
body  surface — the  oral  aperture. 

In  certain  encysted  cells,  such  as  coccidia,  the  cell  wall 
becomes  a  dense  tough  thick  structure  able  to  resist 
drying  and  other  unfavorable  conditions,  and  in  the 
spores  of  bacteria  the  resisting  power  of  the  cell  wall  is 
still  more  highly  developed. 

The  ova  of  tape-worms  are  not  only  surrounded  by  a 
very  tough  cell  wall,  but  this  in  turn  by  a  layer  of  mucous. 

The  mammalian  ovum  has  a  highly  developed  cell 
wall  known  to  histologists  and  embryologists  as  the 
zona  pellucida.  It  is  a  thick,  hyaline  structure,  has 
numerous  projecting  short  filaments,  and  appears  to 
have  many  perforations  large  enough  to  admit  the  head 
of  a  spermatozoon. 

The  highest  development  of  the  cell  wall  is,  however, 
found  in  the  metaphyta  or  higher  plants,  where,  however, 
it  becomes  a  product  of  the  cell  rather  than  an  integral 
part  of  it.  Growing  plant  cells  have  very  thin  walls, 
but  so  soon  as  the  cells  have  attained  their  full  size,  the 
cell  wall  begins  to  increase  in  thickness,  first  by  intussus- 
ception and  later  by  the  deposition  of  new  layers  upon 
the  originally  formed  wall.  In  this  process  cellulose  is 
the  primary  product,  pectin,  lignin,  and  suberin  beirg 
secondary  products  later  deposited.  The  vegetable 


THE    CELL  101 

cell,  in  the  sense  cf  protoplasmic  mass,  thus  comes  to 
lie  loosely  in  a  circumscribed  space,  the  walls  of  which 
are  of  its  own  formation,  but  to  which  it  is  not  indis- 
solubly  bound  and  of  which  it  is  not  itself  a  part. 

Thus  it  appears  that  the  botanist  and  the  zoologist 
mean  somewhat  different  things  when  they  speak  of  the 
cell  wall. 

Many  cells  contain  a  small  rounded  body  known  as 
the  centrosome.  It  is  subject  to  many  variations.  Some- 
times it  appears  shortly  before  cell  division  is  to  take 
place;  sometimes  its  presence  is  constant. 

The  source  and  function  of  this  body  are  unknown. 
It  is  in  some  manner  connected  with  multiplication,  for 
it  always  divides  before  the  other  structures  of  the  cell, 
and  in  many  cases  it  disappears  shortly  after  the  process 
of  cell  division  has  been  completed. 

Vacuoles  are  frequently  present  in  the  cells.  The 
term  is  applied  to  what  seem  to  be  empty  spaces 
in  the  cytoplasm.  They  are  most  frequent  and  largest 
in  vegetable  cells  during  active  growth  and  then  are 
really  drops  of  sap  with  which  the  cytoplasm  distends 
itself  during  the  process  of  nutrition. 

In  animal  cells  similar  vacuoles  are  to  be  found  and 
are  probably  formed  by  the  local  collection  of  the  prod- 
ucts of  digestion  awaiting  assimilation.  In  certain 
of  the  protozoa  these  products  appear  to  collect  in 
preformed  spaces  which  communicate  with  one  another 
so  that  the  contents  of  one  may  be  expelled'into  another. 
In  some  of  these  animals  there  is  a  rhythmical  to-and- 
from  movement  of  the  fluid  from  one  vacuole  to  another, 
contractile  vacuoles.  It  is  supposed  that  the  movement 
of  the  fluid  aids  in  assimilation  through  the  acceleration 
of  cytoplasmic  circulation. 

Vacuoles  not  infrequently  consist  of  reserve  stuffs 
temporarily  stored  in  the  cells.  Of  such  nature  are  the 
globules  of  fat  and  glycogen  so  commonly  found  in  the 
liver  cells  of  man,  and  the  enormous  fatty  globules  in  the 
fat-storing  organs  and  subcutaneous  tissue  of  the  higher 
animals. 


CHAPTER  VI. 
CELL  DIVISION. 

The  most  simple  form  of  reproduction  or  multiplica- 
tion takes  place  through  the  separation  of  the  cell 
into  two  or  more  segments  of  fairly  uniform  size  and 
appearance. 

It  is  usually  preceded  or  accompanied  by  remarkable 
changes  in  the  nucleus  which  have  led  to  its  being  called 
mitosis  by  W.  Schleicher  (1878),  karyokinesis  by  W.  Flem- 
ming  (1882),  or  indirect  cell  division.  The  appearances 
vary  according  to  the  simplicity  or  complexity  of  the 
cellular  structure,  so  that  one  description  cannot  apply 
to  all  cases,  though  an  account  of  what  takes  place  in 
a  typical  cell  may  be  accepted  as  a  type  of  the  process. 

The  cell  that  is  about  to  divide  is  distinctly  larger  than 
its  fellows,  and  its  nucleus  contains  an  excess  of  nuclein 
or  chromatin — hyperchromatosis.  The  cells  formed  by 
division  are  smaller  than  the  normal,  so  that  both  before 
and  after  division  we  see  examples  of  true  cell  growth 
with  actual  increase  of  the  'essential  substances  of  the 
cell. 

It  will  be  convenient  for  our  present  purposes  to  divide 
the  mitotic  changes  into  the  following  phases: 

1.  The  Prophase,  or  Preparation  of  the  Nucleus  for 
Division. — When  an  ordinary  resting  nucleus  stained  by 
the  usual  methods  is  carefully  observed  it  will  be  found 
that  the  nuclear  membrane  is  quite  distinct,  and  that  just 
within  it  there  is  a  more  or  less  well-marked,  slightly 
filamentous  deposit  of  chromatin.  The  remainder  of  the 
nucleus  is  brilliant  and  clear,  with  scattered  threads  of 
102 


CELL   DIVISION 


103 


DIAGRAM  OF  HOMOTYPE  MITOSIS  (Karyokinesis) . 

The  division  of  all  of  the  somatic  and  all  of  the  germinal  cells  is 
accompanied  by  changes  more  or  less  corresponding  with  the  fol- 
lowing diagrams,  except  the  reduction  divisions  of  the  germinal  cells, 
which  occur  by  heterotype  mitosis,  which  will  be  made  clear  by  a 
subsequent  diagram. 


Resting  nucleus 


Prophase 


Teleph 


Any  somatic  or  germinal  eel; 


Stage  preliminary  to  multiplication  of 

any  somatic  or  germinal  cell, 
a.  Formation  of  the  spirem;  disap- 
pearance of  the  nuclear  mem- 
brane, etc. 


6.  Formation  of  enumerable  chro- 
mosomes; appearance  of  cen- 
trosomea  and  nuclear,  spindle. 


TN.     c.  Formation  of  the  polar  and  hypo- 
fc£  \  polar  fields,  and  the  gathering 

«C  I  of     the     chromosomes,     apex 


toward  the  center,  to  form  the 
equatorial  plate  and  "aster." 


Each  chromosome  splits  longitudinally, 
the  corresponding  parts  maintain- 
ing their  normal  positions  in  the 
equatorial  plate.  The  hrlf-chro- 
mosomes  are  of  equal  valence. 

a.  The  halves  of  each  divided  chro- 

mosome separate  and  turn 
toward  opposite  polar  fields. 
Thus  the  future  nuclei  receive 
exactly  one-half  of  the  chro-' 
matin,  in  the  form  of  one-half 
of  each  chromosome. 

b.  The       separated       chromosomes 

gather  at  opposite  ends  of  the 
cell  about  the  polar  and  hypo- 
polar  fields  to  form  the  "amphi- 
aster." 


The  division  of  the  cytoplasm  after 
separation  of  the  daughter-nuclei, 
and  the  loss  of  the  star-like  arrange- 
ment of  the  chromosomes. 


Complete  division  of  the  cell  into  two 
of  equal  size  and  value. 


Fid.  31. 


104  BIOLOGY:  GENERAL  AND  MEDICAL 

chromatin.  If  a  nucleolus  be  present,  it  appears  as  a  mi- 
nute distinct  dot.  As  the  chromatin  increases  in  quantity 
prior  to  division,  it  comes  to  fill  the  greater  part  of  the 
nucleus  and  the  nucleolus  disappears.  Soon  the  chroma- 
tin  increases  in  filamentousness  until  a  single  long,  some- 
what spirally  coiled  thread — the  spirem — is  formed.  When 
examined  under  most  appropriate  conditions  this  thread 
has  a  fuzzy  appearance,  which  is  supposed  to  depend 
upon  an  incomplete  differentiation  of  the  chromatin  from 
the  linin,  which  clings  to  it.  Soon,  however,  the  linin 
separates  completely,  after  which  it  is  observed  that  the 
chromatin  no  longer  forms  a  single  thread,  but  has  broken 
into  a  number  of  segments  of  uniform  length,  which  are 
the  chromosomes.  These  bodies,  first  described  in  1888 
by  Waldeyer,  are  of  great  interest  from  many  points  of 
view,  and  are  believed  by  Weismann  and  others  to  endow 
the  offspring  of  the  cell  with  its  chief  hereditary  impulses. 
The  number  of  chromosomes  is  usually  the  same  in  all  of 
the  cells  of  the  same  organism,  though  it  varies  in  different 
kinds  of  organisms,  and  in  the  two  sexes  of  any  kind  of 
organism.  When,  as  in  the  cells  of  vertebrates,  and  es- 
pecially mammals,  the  number  of  chromosomes  is  great, 
there  is  much  difficulty  in  counting  them.  Thus,  the 
somatic  cells  of  human  beings,  oxen,  and  guinea-pigs  are 
usually  said  to  have  sixteen  chromosomes;  those  of  the 
•mouse,  salamander,  and  trout,  twenty-four.  Mont- 
gomery, who  painstakingly  studied  the  spermatocytes  of 
man,  thought  that  they  contained  twenty,  twenty-two, 
or  twenty-four  chromosomes;  but  Winiwarter,  whose 
work  is  more  recent,  believes  that  there  are  forty-seven 
in  the  human  male  and  forty-eight  in  the  human  female. 
In  complex  organisms  there  is  also  a  difference  in  shape 
between  the  chromosomes  of  the  somatic  and  germinal 
cells,  by  which  the  latter  are,  at  one  time,  made  to  appear 
twice  the  number  of  the  former,  and  at  another  time  are 
reduced  to  one-half  the  number  of  the  former. 

In  cells  in  which  no  centrosome  is  usually  visible,  that 


CELL   DIVISION  105 

organ  now  makes  its  appearance  surrounded  by  a  con- 
densed or  highly  granular  area  of  cytoplasm  known  as 
the  attraction  sphere,  which  is  to  become  the  "polar  field." 

During  these  changes  the  nuclear  membrane  has  im- 
perceptibly disappeared,  leaving  the  chromosomes  free 
in  the  cytoplasm.  The  chromosomes  in  the  meantime 
have  uniformly  assumed  a  V-shape  and  arranged  them- 
selves about  the  polar  field  with  the  apices  toward  the 
centre.  It  now  makes  considerable  difference  from  which 
direction  one  views  the  groups  of  chromosomes,  for  at  the 
same  time  that  this  adjustment  has  been  in  progress  the 
centrosome  has  divided  and  its  halves  have  gradually  sepa- 
rated, so  that  we  have  now  a  "polar  field"  and  a  "hypo- 
polar  field,"  each  occupied  by  a  centrosome  from  one  to 
the  other  of  which  delicate  filaments  of  linin  extend,  form- 
ing a  peculiar  object  known  as  the  "nuclear  spindle." 

If  the  group  of  chromosomes  be  viewed  from  a  direc- 
tion corresponding  to  the  polar  or  hypopolar  fields,  one 
sees  those  bodies  with  their  apices  directed  toward  the 
centre,  their  ends  outward,  forming  an  appearance  some- 
times compared  to  a  wreath,  but  usually  to  a  star,  and 
spoken  of  as  the  aster  or  "mother-star."  If,  on  the  other 
hand,  they  are  viewed  from  a  point  at  right  angles  to  the 
axis  of  the  nuclear  spindle,  it  will  be  found  that  they  all 
lie  in  a  plane  perpendicular  to  the  axis  of  the  spindle, 
known  as  the  "equatorial  plate." 

2.  The  Metaphase,  or  Division  of  the  Chromosomes. — 
The  metaphase  begins  with  a  longitudinal   division  of 
each  chromosome.     Here  we  find  the  first  indication  of 
what  is  to  be  the  final  outcome  of  the  process.     Each 
chromosome  has  given  rise  to  two  smaller  chromosomes 
of  equal  value,  and  one-half  of  each  is  to  enter  into  each 
of   the   newly  forming   nuclei.     The  chromosomes   next 
manifest  a  change  of  position. 

3.  Anaphase,  or  the  Separation  of  the  Chromosomes. — 
Beginning  at  the  centre,  the  apical  portion  of  each  half- 
chromosome   moves   from   its   fellow   and   directs   itself 


106  BIOLOGY:  GENERAL  AND  MEDICAL 

toward  one  pole  of  the  nuclear  spindle,  until  there  has  been 
an  exact  separation  of  the  nuclear  material,  one-half 
having  gradually  moved  into  the  polar  field,  the  other 
half  into  the  hypopolar  field. 

This  results  in  the  formation  of  a  double  figure,  the 
amphiaster,  or  "daughter-stars."  Each  exactly  resembles 
the  aster  or  mother-star  except  that  its  size  is  smaller. 

During  this  phase  of  the  nuclear  changes  the  cytoplasm 
shows  a  constriction  in  the  line  of  the  equatorial  plate 
which  progressively  deepens,  so  that  about  the  time  the 
nuclear  changes  are  perfected  the  separation  into  two 
cells  is  also  completed. 

4.  The  Telophase,  or  the  Transformation  of  Each  Daughter- 
Star  into  a  Perfect  Nucleus. — This  is  accomplished  by  a 
succession  of  events  corresponding  to  those  described  as 
characterizing  the  prophase,  except  that  they  occur  in 
the  reverse  order — i.e.,  the  chromosomes  abandon  their 
star-like  formation,  adjust  themselves  to  form  a  spirem, 
become  mixed  with  the  linin  filaments,  and  lose  their 
distinctness  until  the  usual  nuclear  appearances  are  re- 
sumed. 

This  general  outline  of  the  cell  division  is,  however, 
subject  to  many  modifications,  necessitated  by  the  sim- 
plicity and  complexity  of  the  cells  and  the  number  into 
which  they  divide. 

In  those  cells  in  which  no  nuclei  can  be  discovered  by 
existing  methods  of  examination,  no  nuclear  changes 
can  be  associated  with  division.  The  cell  having  grown 
to  a  certain  size,  appears  to  separate  into  two  or  more 
equal  parts,  each  of  which  takes  on  an  independent 
existence.  The  structure  of  certain  cells,  as  the  bacteria, 
is  still  controversial.  Some  believe  them  to  have  large 
nuclei  and  see  in  them  appearances  analogous  to  those 
of  karyokinesis  at  the  time  of  division.  It  is  usual, 
however,  to  describe  the  division  of  these  organisms 
as  taking  place  by  direct  division,  i.e.,  simple  fission 
without  karyokinesis. 


CELL    DIVISION 


107 


Concerning  certain  nucleated  cells  of  the  metazg,a  we 
are  also  still  in  doubt.  Thus,  Arnold  and  others  have 
described  simple  fission  or  direct  division  in  certain 
lymphoid  cells  of  man,  but  whether  their  observations 
would  bear  the  test  of  more  improved  methods  of 
examination  is  not  yet  determined.  The  more  perfect 
the  methods  of  staining  and  examining  the  cells,  the 
more  strongly  we  become  convinced  that  there  is  no 
cell  division  independent  of  antecedent  changes  analo- 
gous to  karyokinesis. 

When  the  cell  divides  into  many  segments,  the  karyo- 
kinetic  process  is  of  necessity  modified  to  conform  to  the 
requirements  of  the  case.  The  primary  division  appears 
to  take  place  in  the  centrosome.  If  two  are  formed,  there 


o.  b 

FIG.  32. — Direct  division  of  lymphocytes  of  a  frog.     (Arnold.) 

will  be  a  simple  nuclear  spindle  and  the  nuclear  material 
being  equally  divided  and  drawn  toward  them,  two  new 
nuclei  and  two  new  cells  will  result;  if  there  are  four 
divisions  of  the  centrosome,  there  will  be  two  nuclear 
spindles  and  four  new  nuclei;  if  a  greater  number  of  divi- 
sions of  the  centrosome,  a  still  more  complex  arrange- 
ment of  spindles  and  a  still  greater  number  of  nuclei  will 
be  formed.  In  the  sporozoa  in  which  great  numbers  of 


108  BIOLOGY:  GENERAL  AND  MEDICAL 

minute  cells  or  spores  arise  through  the  division  of  one 
large  one,  the  chromosome  formation  and  nuclear  spind- 
les are  theoretically  present  but  not  actually  visible. 

In  morbid  conditions  the  process  of  cell  division  may 
miscarry  in  a  variety  of  ways.  Thus,  in  the  event  of  the 
separation  of  the  nuclear  material  being  imperfect  and 
incomplete,  a  large  lobulated  nucleus  may  be  formed;  in 
case  the  nuclear  divisions  are  completed  without  accom- 
panying division  of  the  cytoplasm,  giant  cells  may  be 
formed. 


CHAPTER  VII. 
THE  HIGHER  ORGANISMS. 

A  superficial  acquaintance  with  the  structure  of 
familiar  living  things  is  sufficient  to  enable  a  casual 
observer  to  arrrange  them  in  a  series  in  which  they  pass 
from  the  simple  unicellular  to  more  and  more  complex 
multicellular  forms.  This  does  not  necessarily  mean 
that  it  is  possible  to  follow  the  individual  steps  through 
which  the  complexity  was  attained,  nor  does  it  imply 
ability  to  interpret  the  purpose  of  the  many  diverse 
forms  to  which  this  complexity  has  arrived.  Such 
problems  are  reserved  for  consideration  in  a  future 
chapter  where  it  is  hoped  that  they  may  be  presented  in 
a  reasonable  form. 

It  is  difficult  to  avoid  the  conviction  that  complexity 
of  structure,  with  the  many  differentiations  and  adap- 
tations it  embraces,  is  of  great  importance  to  living 
substance  in  improving  the  conditions  of  life  and  better 
adapting  it  to  the  exigencies  of  existence,  yet  simply 
and  complexly  organized  forms  of  life  exist  in  nature  side 
by  side,  both  equally  able  to  persist.  The  idea  of  pur- 
pose in  progress  is,  therefore,  compelled  to  give  place  to 
the  opinion  that  what  is  seen  in  this  evolution  is  but 
evidence  of  the  fact  that  life  is  cyclical  and  subject  to 
changes  by  which  modification  is  inevitable.  The 
inutility  of  complexity  is  further  exemplified  by  the 
fact  that  many  highly  complex  and  differentiated  or- 
ganisms have  become  extinct,  while  many  simple  organ- 
isms have  persisted. 

With  increasing  complexity  of  structure  come  more 
-complicated  conditions  of  life  which  make  it  more  and 
109 


110  BIOLOGY:  GENERAL  AND  MEDICAL 

more  difficult  for  the  highly  evolved  organism  to  survive 
in  the  general  struggle  for  existence. 

The  foreshadowing  of  organs — elementary  organs — 
are  to  be  found  among  the  protozoa  and  indicate  that 
these  lowly  organisms  are  subject  to  perhaps  as  much 
specialization  as  is  compatible  with  their  unicellular 
simplicity. 

Thus  from  the  protamoeba  and  amoeba  whose  shapes 
can  scarcely  be  regarded  as  fixed,  and  which  consist  of 
plastic  masses  of  protoplasmic  jelly,  we  pass  to  higher 
amosba  that  cover  themselves  with  more  or  less  elaborate 


FIG.  33. — Amoeba  proteus  (magnified).  The  largest  sphere  is  the  contractile 
vacuole;  the  smaller  is  the  nucleus.  A  large  diatom  is  seen  on  the  left,  and 
numerous  paraplasmic  granules  are  scattered  through  the  protoplasm. 
(Masterman.) 

cases  composed  of  minute  particles  of  mineral  substance, 
and  then  to  the  foraminifera  in  which  the  body  substance 
is  surrounded  by  fantastic  calcareous  shells  through 
fixed  openings  in  which  the  pseudopods  may  be  pro- 
truded. We  next  pass  to  other  forms  in  which  the 
development  of  the  cell  wall  occurs  as  a  hyaline  but  rigid 
cuticle  giving  permanent  form  to  the  organism  as  in 
Paramcecium,  Vorticella,  Epistylus,  Stentor,  etc. 

In  the  flagellates  and  ciliates  we  find  the  cuticular 
cell,  wall  perforated  by  many  minute  openings  through 
which  special  rigid  protoplasmic  threads  project  for 


THE   HIGHER   ORGANISMS  111 

purposes  of  motion  and  locomotion.  Many  of  these 
forms  are  further  provided  with  fixed  oral  openings 
surrounded  by  cilia  directing  currents  into  a  tubular 
aperture  terminating  abruptly  in  the  cytoplasm,  but 
acting  as  a  primitive  alimentary  canal. 

Likewise  we  find  the  desmids  and  diatoms  among  the 
vegetable  cells  provided  with  complex  cellulose  enve- 
lopes and  silicious  skeletons,  and  foraminifera  and  radi- 


c— 


FIG.  34. — Carchesium.  Showing  beginning  specialization.  (Greenwood.) 
The  path  which  the  food  takes  is  represented  by  dots,  a  (circular  round 
marks)  represents  the  position  of  storage;  6  (crosses)  represents  the  position  of 
rest;  c  (dots),  the  region  of  the  later  changes. 

olaria  among  animals  with  wonderfully  elaborate  sili- 
cious skeletons  which  endow  the  organism  with  definite 
form  and  increase  its  rigidity. 

These  primitive  specializations  find  their  analogues 
in  the  dermal  coverings,  the  limbs  and  fins,  etc.,  of  the 
higher  animals. 

It  is  self-evident,  however,  that  an  organism  composed 
of  a  single  cell  is  less  well  able  to  increase  its  complexity 


112 


BIOLOGY:  GENERAL  AND  MEDICAL 


than  one  composed  of  many  cells,  groups  of  which  can  be 
set  aside  for  special  purposes. 

Composite  structure,   however,   does  not  necessarily 
imply  complicated  structure;  it  only  affords  opportunity 


Fia.  35. — Various  forms  of  Radiolaria.     (After  Haeckel.) 
1,    Ethmosphoera   siphomophora;    2,  Actinomma    incrme;    3,  Acanthometra 
xiphicantha;  4,  Arachnosphosra  oligacantha;  5,  Cladococcus  viminalis. 

for  it.  Thus  we  find  lowly  forms  of  life  sometimes  made 
up  of  congeries  of  similar  cells  whose  cytoplasm  has 
become  united  into  a  general  undifferentiated  mass  or 
plasmodium.  Such  formations  occur  in  both  the  animal 


THE    HIGHER    ORGANISMS  113 

and  vegetable  kingdoms,  but  in  neither  do  such  plas- 
modia  show  the  least  tendency  to  differentiation  of  cells 
into  tissues  such  as  compose  the  more  complex  organisms. 

The  size  of  the  organism  bears  very  little  relation  to  its 
complexity.  It  is  true  that  size  affords  opportunity  for 
differentiation,  but  some  of  the  undifferentiated  plas- 
modia  are  large  enough  to  be  measured  in  centimetres, 
while  some  highly  differentiated  and  exceedingly  com- 
plex organisms  like  rotifers  are  visible  only  through  the 
microscope. 

In  endeavoring  to  ascend  from  the  simple  to  the  com- 


FIG.  36. — Orbitolites.      Ideal    representation    of    a    disc    of    complex     type. 

(Carpenter.) 

plex  we  are  confronted  by  certain  conditions  the  full 
force  of  which  cannot  be  felt  until  the  chapter  upon 
Divergence  has  been  perused.  One  of  the  most  impor- 
tant of  these  is  the  extinction  of  many  of  the  intermedi- 
ate forms,  so  that  just  when  we  seem  to  be  comfortably 
started  upon  a  series  of  easy  anatomical  gradations,  we 
are  obliged  to  skip  a  number  of  steps  and  endeavor  to 
fill  them  with  imaginary  forms.  Another  and  even 
greater  difficulty  is  found  in  the  fact  that  the  process  of 
differentiation  is  not  uniform,  specialization  in  certain 


114  BIOLOGY:  GENERAL'  AND  MEDICAL 

directions  enabling  us  to  create  a  fairly  continuous  series 
in  that  particular  line,  though  the  members  of  the  series 
find  no  correspondence  in  other  lines. 

This  discrepancy  sometimes  embarrasses  systematic 
writers  as  they  endeavor  to  perfect  the  classification  of 
living  things,  for  a  classification  based  upon  the  resem- 
blances of  adult  organisms  might  differ  widely  from  one 
based  upon  embryological  resemblances. 


FIG.  37. — Fossil  Diatomacese,  etc.,  from  Oran.  a,  a,  a,  Coscinodiscus; 
b,  b,  b,  Actinocyclus;  c,  Dictyochya  fibula;  d,  Lithasteriscus  radiatus;  e,  Spon- 
golithis  acicularis;  /,  /,  Grammatophora  parallela  (side  view);  g,  g,  Gramma- 
tophora  angulosa  (front  view).  (Carpenter.) 

In  the  discussion  before  us  greater  success  will  prob- 
ably accrue  if  the  living  things  are  compared  without 
fixed  ideas  as  to  their  precise  position  in  the  zoological 
or  botanical  classifications. 

From  the  plas medium  in  which  association  of  many 
cells  is  followed  by  obliteration  of  the  identity  of  each, 
and  in  which  it  is  difficult  to  see  that  the  association 
subserves  any  useful  purpose  or  predisposes  to  the 
occurrence  of  complexity  through  increase  of  size  or 


THE    HIGHER   ORGANISMS  115 

number  of  individuals,  we  next  pass  into  congeries  of 
cells  in  which  the  identity  of  each  is  preserved,  though 
no  apparent  utility  is  subserved  thereby.  This  is  seen, 
for  example,  in  Spirogyra,  where  hundreds  of  organisms 
may  adhere  to  one  another,  in  Epistylus,  Carchesium, 


FIG.  38. — Microgromia  socialis.  A,  Colony  of  individuals  in  extended  state, 
some  of  them  undergoing  transverse  fission;  B,  colony  of  individuals  (some  of 
them  separated  from  the  principal  mass)  in  compact  state;  C,  D,  formation  and 
escape  of  swarm-spore,  seen  free  at  E.  (Carpenter.) 

and  other  infusoria  where  organisms  resulting  from  the 
division  of  the  parent  cell  remain  attached  by  their 
pedicles  to  a  common  stalk.  A  less  fixed  colonial  aggre- 
gation is  seen  in  Microgromia  socialis,  where  individuals 
remain  united  for  a  time  by  their  pseudopods,  sometimes 


116  BIOLOGY:  GENERAL  AND  MEDICAL 

more  or  less  distant  from  one  another,  and  connected 
only  by  delicate  protoplasmic  filaments,  sometimes 
forming  compact  masses.  Any  individual  seems  able 
to  withdraw  from  the  union  to  found  a  new  colony, 
though  this  is  usually  observed  to  follow  division  of  one 
of  the  members  whose  descendant,  assuming  a  modified 
form,  escapes  from  the  cell  capsule  in  which  it  was  born 
and  deserts  the  colony. 

In  many  cases  the  primary  purpose  of  cell  association 
appears  to  be  founded  upon  reproductive  advantages 


FIG.  39. — Magosphsera  planula,  a  ciliated  colonial  protozoan.     (After  Haeckel.) 

thus  afforded,  for  one  of  the  first  structural  specializa- 
tions to  be  observed  among  the  lowly  forms  of  life  con- 
sists of  special  reproductive  cells. 

We  find  that  with  the  exception  of  the  most  lowly 
organisms  of  both  animal  and  vegetable  forms,  repro- 
duction takes  place  either  exclusively  or  most  advan- 
tageously when  the  newly  formed  individual  or  indivi- 
duals receive  substance  directly  or  indirectly  from  two 
individuals  of  its  kind  (amphimixis).  Emphasis  will 
be  laid  upon  the  importance  and  significance  of  this  in 
the  chapter  upon  Reproduction. 


THE    HIGHER    ORGANISMS 


117 


So  soon  as  it  becomes  clear  that  the  purpose  of  cell 
association  is  reproduction,  and  so  soon  as  it  can  be 
determined  that  among  the  associated  cells  some  are 


FIG.  40. — Volvox  globator.  A,  the  entire  colony,  surface  view,  showing  the 
biflagellate  zooids  and  several  daughter-colonies  swimming  freely  in  the  interior; 
the  latter  are  produced  by  the  repeated  fission  of  non-flagellate  reproductive 
zooids  (a).  B,  the  same  during  sexual  maturity,  showing  spermaries  from 
the  surface  (spy),  in  profile  (spy')  and  after  complete  formation  of  sperms 
(spy");  and  ovaries  from  the  surface  (ovy,  ovy",  ovy'",)  and  in  profile  (ovyr). 
C,  four  zooids  in  optical  section,  showing  cell  wall,  nucleus,  contractile  vacuole, 
with  adjacent  pigment-spot,  and  flagella  (ft).  D'-D5,  stages  in  the  formation 
of  a  colony  by  the  repeated  binary  fission  of  an  asexual  reproductive  zooid.  E, 
a  ripe  spermary.  F,  a  single  sperm,  showing  pigment-spot  (pg)  and  flagella 
(ft).  G,  an  ovary  containing  a  single  ovum  surrounded  by  several  sperms. 
H,  oospenn  enclosed  in  its  spinose  cell  wall.  (After  Kirchner;  B-H  after 
Cohn.) 


destined  to  perform  the  reproductive  function  alone, 
the  advantage  of  the  association  becomes  clear  for  the 
reproductive  cells  may  then  be  relieved  of  other  func- 
tions through  the  vicarious  activity  of  the  other  cells. 


118 


BIOLOGY:  GENERAL  AND  MEDICAL 


FIG.  41. — Hydra.  A,  Vertical  section  of  the  entire  animal,  showing  the 
body  wall  composed  of  ectoderm  (ect)  and  endoderm  (end),  enclosing  an  enteric 
cavity  (ent.  cav),  which,  as  well  as  the  two  layers,  is  continued  (ent.  caw')  into 
the  tentacles,  and  opens  externally  by  the  mouth  (mtli)  at  the  apex  of  the 
hypostome  (hyp).  Between  the  ectoderm  and  endoderm  is  the  mesogloea  (msgl), 
represented  by  a  black  line.  In  the  ectoderm  are  seen  large  (ntc)  and 
small  (ntc')  nematocysts;  some  of  the  endoderm  cells  are  putting  out  pseudopods 
(psd),  others  flagella  (fl).  Two  buds  (bd1,  bd2)  in  different  stages  of  development 
are  shown  on  the  left  side,  and  on  the  right  a  spermary  (spy)  and  an  ovary  (ory) 
containing  a  single  ovum  (ov).  B,  portion  of  a  transverse  section  more  highly 
magnified,  showing  the  large  ectoderm  cells  (ect)  and  interstitial  cells  (int.  c); 
two  cnidoblasts  (cnbl)  enclosing  nematocysts  (ntc),  and  one  of  them  produced 
into  a  cnidocil  (cnc) ;  the  layer  of  muscle  processes  (m.pr)  cut  across  just  external 
to  the  mesoglu?a  (msgl);  endoderm  cells  (end)  with  large  vacuoles  and  nuclei 


THE    HIGHER    ORGANISMS  119 

A  beautiful  example  of  this  is  shown  in  the  case  of 
Volvox  giobator.  This  little  plant  begins  its  life  history 
as  a  hollow  sphere  composed  of  cells  which  at  first  show 
no  essential  differences,  are  somewhat  polyhedral  in 
form,  and  separated  from  one  another  by  a  hyaline 
material.  About  the  whole  mass  there  is  a  distinct 
membranous  formation.  Among  the  component  cells 
one  larger  than  the  others  can  early  be  distinguished. 
As  the  total  number  of  cells  continues  to  increase  and  the 
hyaline  intercellular  substance  likewise  to  increase,  this 
particular  cell  enlarges  and  then  divides  by  successive 
segmentations  until  as  many  as  sixty-four  or  even  one 
hundred  and  twenty-eight  similar  cells  have  been  formed. 
These  are  the  reproductive  cells.  As  their  fertiliza- 
tion is  accomplished,  they  project  into  the  central  cavity 
of  the  hollow  organism,  eventually  divide  into  many 
cells,  of  which  a  similar  hollow  sphere  is  formed  which 
remains  in  the  body  of  the  parent  until  upon  its  disso- 
lution a  number  of  young  spheres  are  simultaneously 
set  free  to  repeat  the  cycle  described. 

We  have  no  right  to  regard  anything  that  takes  place 
in  nature  as  accidental  simply  because  we  see  no.purpose 
in  it  or  reason  for  it.  We  must  not,  therefore,  hastily 
conclude  that  loose  combinations  of  organisms,  such  as 
we  see  in  Streptococcus,  Spirogyra,  Epistylus,  Carches- 
ium,  Microgromia,  etc.,  are  unimportant  to  the  organisms 
concerned. 

By  having  many  cells  a  division  into  somatic  and 
germinal  forms  is  quickly  followed  by  the  development 
of  the  former  as  the  hosts  or  caretakers  of  the  latter. 
Such  a  primary  differentiation  is  in  turn  soon  followed 


(nu),  pseudopods  (psd).  and  flagella  (fl).  The  endoderm  cell  to  the  right  has 
ingested  a  diatom  (a),  and  all  enclose  minute  black  granules.  C,  two  of  the 
large  ectoderm  cells,  showing  nucleus  (nu)  and  muscle  process  (m.  pr).  D,  an 
endoderm  cell  of  H.  viridis,  showing  nucleus  (nu),  numerous  chromatophores 
(chr),  and  an  ingested  nematocyst  (ntc).  E,  one  of  the  larger  nematocysts 
with  extruded  thread  barbed  at  the  base.  F,  one  of  the  smaller  nemotocysts. 
G,  a  single  sperm.  (A,  B,  C,  E  after  Parker,  "Lessons  in  Elementary  Biology"; 
D  after  Lankester;  F  and  G  after  Howes.) 


120  BIOLOGY:  GENERAL  AND  MEDICAL 

by  the  development  of  the  soma  or  body  cells  into 
groups  set  aside  for  special  functions,  such  groups  be- 
coming more  and  more  differentiated  by  the  continu- 
ally increasing  complexity.  The  movement  once  started 
advances  rapidly  after  the  void  between  the  unspecial- 
ized  unicellular  and  the  definitely  specialized  multi- 
cellular  forms  of  life  has  once  been  bridged. 

With  the  increasing  complexity  of  the  soma  come 
differentiations  which  seem  to  affect  it  alone,  but  react 
upon  the  germ.  Thus,  for  example,  one  might  doubt 
that  the  possession  of  ornamental  appendages  could 
in  any  way  benefit  the  germ,  but  analysis  of  the  question 
will  show  that  it  is  one  of  the  circumstances  by  which 
the  good  of  the  host  is  advanced  and  so  is  of  impor- 
tance to  the  germ.  Indeed  the  evolution  of  living  matter 
depends  in  large  measure  upon  the  reciprocal  relation- 
ships of  the  soma,  and  the  germ. 

Continuing  to  study  increasing  complexity,  we  pass 
from  those  examples  in  which  the  combination  of  cells 
appeared  to  be  temporary  and  loose,  any  detached 
member  of  the  colony  being  able  to  maintain  itself  and 
eventually  to  establish  a  new  colony,  as  in  Microgromia, 
Carchesium,  and  Epistylus,  to  such  as  Volvox  where  the 
combinations  are  fixed  and  permanent  and  individual 
members  detached  from  the  whole  languish  and  die. 

Among  the  metazoa  we  find  no  fixed  combinations  of 
undifferentiated  cells  forming  single  organisms.  In 
the  very  lowest  we  find  definite  disposition  of  the  cells — 
a  regular  arrangement — to  subserve  a  definite  function. 
Thus  among  the  sponges  and  the  hydras  we  find  the 
cells  disposed  in  two  chief  layers  whose  functions  differ 
more  than  their  general  appearance.  The  fresh-water 
hydra  consists  of  two  layers  of  cells,  an  outer  forming 
the  ectoderm  or  covering  layer  and  an  inner  forming  the 
entoderm  which  is  digestive  and  nutritive.  The  entire 
animal,  its  body  as  well  as  its  tentacles,  and  any  budding 
offspring  that  may  be  attached  to  it,  is  composed  of 
these  two  simple  layers.  Between  them  is  a  narrow  in- 


THE    HIGHER    ORGANISMS 


121 


terval  usually  without  cells,  but  sometimes  containing 
cells  of  amoeboid  character  that  have  crawled  in  between 
the  other  cells.  This  space  with  whatever  cells  it  con- 
tains is  called  the  mesenchyme.  In  such  a  simple  organ- 
ism, the  chief  office  of  the  outer  cells  of  the  body  is  to  pro- 
tect it  by  forming  an  elementary  or  primitive  cuticle. 
The  office  of  the  cells  of  the  inner  layer  is  to  dissolve  nutri- 


FIG.  42. — Diagram  of  simple  type  of  sponge,  c,  cloaca;  ch,  chambers,  lined 
with  flagellate  entoderm;  e.p,  external  pores;  i.p,  internal  pores;  mes,  mesen- 
chyma;  o,  osculum;  r.c,  radiating  canals;  ec,  ectoderm;  en,  entoderm.  In  the 
adult  sponge  the  canals  and  flagellate  chambers  become  much  more  complex 
than  figured  here.  (Galloway.) 


tious  particles  brought  into  the  body  cavity  by  the  ten- 
tacles. Through  their  action  the  fluid  contained  in  the 
body  cavity  becomes  nutritious  enough  to  enable  the 
cells  to  live  by  absorbing  it,  the  inner  cells  passing  it  to 
their  next  neighbors  of  the  outer  layer,  or  permitting  it 
to  transfuse  into  the  mesenchyme  from  which  it  reaches 
them. 


122  BIOLOGY:  GENERAL  AND  MEDICAL 

Among  the  porifera  or  sponges,  as  among  the  higher 
coelenterates,  the  mesenchyme  increases  in  importance 
so  that  we  have  three  tissues,  the  ectoderm,  the  entoderm, 
and  the  mesenchyme,  which  may  be  regarded  as  the  start- 
ing point  of  all  the  future  differentiations  for  even  among 
the  highest  animals,  and  in  man  himself,  the  tissues  are 
still  divided  into  those  springing  from  one  or  the,  other 
of  these  three  structures,  the  outer  of  which  is  the  source 
of  the  integuments,  the  inner  the  source  of  the  organs 
of  digestion,  and  the  middle  the  source  of  the  organs  of 
support  and  locomotion. 

The  increasing  structural  complexity  appears  to  be 
largely  a  matter  of  necessity.  As  the  number  of  cells 
increases,  and  specialization  of  these  cells  is  gradually 
effected,  it  becomes  inevitable  that  all  should  not  be 
favorably  situated  with  reference  to  the  source  of  nu- 
trition, so  that  some  means  must  be  provided  for  con- 
veying suitable  pabulum  to  them.  As  more  of  the 
nutritive  pabulum  is  required,  greater  perfection  of  the 
organs  of  motion  or  prehension  must  be  developed  in 
order  that  it  may  be  supplied,  and  greater  perfection 
of  the  digestive  organs  becomes  necessary  that  none  of 
it  wastes.  As  the  differentiation  of  parts  becomes  more 
and  more  perfect,  and  their  interdependence  increases, 
means  by  which  one  part  may  communicate  with  another 
appears  in  the  form  of  the  nervous  system. 

Should  we,  however,  seek  to  explain  complexity  of 
structure  by  conditions  intrinsic  in  the  organism  itself, 
and  solely  -along  the  lines  suggested  in  the  preceding 
paragraphs,  we  must  inevitably  fail.  Such  conditions 
would  determine  a  uniform  evolution  of  the  developing 
substance  and  not  result  in  the  diversity  of  structure 
found  among  the  many  phyla  of  plants  and  animals. 
The  chief  sources  of  structural  modification  lie  outside 
of  the  organism  in  its  environment  as  will  be  explained 
in  a  subsequent  chapter. 

We  will  now  endeavor  to  trace  each  of  the  important 
systems  of  organs  back  to  its  inception,  remembering 


THE    HIGHER   ORGANISMS  123 

that  in  most  cases  their  homologues  can  be  found  in 
exceedingly  elementary  forms  of  life. 

The  Reproductive  System. — It  has  already  been  sug- 
gested that  reproduction  is  at  the  very  foundation  of 
cell  association,  and  therefore  is  one  of  the  first,  if  not 
the  first,  specialization  of  the  composite  organism.  At 
first  it  is  so  indefinite  that  any  cell  may  apparently  take 
on  the  function  when  the  appropriate  moment  is  at 
hand  and  the  germinal  cells  seem  to  be  widely  scattered 
among  the  somatic  cells,  but  gradually  they  become 
collected  into  groups — gonads — which  form  the  founda- 
tion of  the  reproductive  organs.  A  future  chapter  will 
be  devoted  to  the  subject  of  reproduction. 

The  Digestive  System. — Unicellular  organisms  nourish 
themselves  as  they  perform  all  other  functions,  through 
activities  of  their  own.  Nutritious  materials  absorbed 
or  ingested  into  their  substance  meet  solution  by  diges- 
tion and  subsequent  assimilation  in  the  cell,  the  residuum 
being  extruded.  Among  the  colonial  protozoa  the  cells 
may  or  may  not  retain  this  independence.  Thus  in  epis- 
tylus  and  carchesium,  we  have  no  re.ason  to  suppose 
that  any  individual  contributes  to  the  support  of  any 
other.  In  microgromia,  however,  it  may  be  that  cells  that 
are  more  successful  in  gathering  up  nutritious  matter 
give  up  some  of  the  proceeds  to  the  less  successful  cells 
through  their  protoplasmic  connections.  Among  vege- 
table cells  community  of  interest  in  regard  to  nutrition 
appears  very  early  and  assumes  great  importance  for 
what  is  seen  among  the  most  lowly  forms  of  animal  life, 
i.e.,  the  transmission  of  nutritious  pabulum  from  cell  to 
cell  persists  even  among  the  highest  forms  of  vegetable 
life. 

Among  the  elementary  animal  composits  the  differ- 
entiation of  function  is  not  sufficient  to  prevent  almost 
any  cell  yielding  to  primitive  tendencies.  Thus  among 
the  porifera  the  digestive  cells  constantly,  and  in  hydra 
Jiot  infrequently,  take  useful  particles  directly  into  their 
own  substance.  This  is  particularly  interesting  in. 


124  BIOLOGY:  GENERAL  AND  MEDICAL 

hydra  where  a  well-formed  digestive  cavity  is  present. 
Large  objects  entering  the  oral  orifice  are  dissolved  in 
the  gastric  cavity  by  enzymes  derived  from  the  ento- 
dermal  cells,  and  the  assimilable  products  of  this  diges- 
tion are  absorbed.  Small  particles  are  liable  to  be 
seized  upon  by  the  cells  and  treated  by  the  more  primi- 
tive process  of  phagocytosis  or  intracellular  digestion. 
The  more  complex  crelenterates  continue  to  display 
more  or  less  of  this  phagocytosis  of  the  entodermal 
cells,  but  as  we  ascend  into  the  phylum  Annulata  the 
specialization  of  the  entodermal  cells  as  digestive  enzyme 
producers  becomes  so  distinct  that  it  disappears  not  to 
be  seen  again  among  the  higher  animals. 

The  primitive  digestive  system  exemplified  by  the 
gastric  cavity  of  hydra,  in  which  the  oral  orifice  sub- 
serves the  double  purpose  of  mouth  and  anus,  finds  an 
improvement  in  the  echinoderms  and  worms  by  the 
addition  of  a  separate  anus  at  its  aboral  end.  The  food 
ingested  now  passes  slowly  through  the  enteron  as 
Haeckel  has  •  called  this  primitive  stomach-intestine, 
being  digested,  during  its  passage,  the  residuum  being 
discharged  from  the  anus.  The  further  specializations 
are  for  the  most  part  in  the  improvement  and  localiza- 
tion of  the  digestive  forces.  The  enteron  becomes 
more  and  more  tubular  and  gradually  separates  itself 
into  a  mouth  for  mastication,  which  is  provided  with  a 
variety  of  different  appendages,  teeth  for  crushing  and 
comminuting,  salivary  glands  for  moistening  the  food, 
etc.,  a  gullet  through,  which  the  food  enters  the  main 
digestive  viscus,  a  stomach,  and  an  elongated  intestine 
from  which  absorption  of  the  products  of  digestion  may 
take  place. 

Instead  of  the  digestion  being  partly  intracellular, 
the  cells  are  specialized  as  enzyme  producers,  arranged 
in  glands  by  which  digestive  juices  are  poured  into  the 
alimentary  canal  and  mixed  with  the  food  so  that  com- 
bined maceration,  solution,  and  digestion  may  prepare  the 
assimilable  substances  for  absorption  from  the  intestine. 


THE   HIGHER   ORGANISMS  125 

Gradually  the  glandular  organs  are  perfected  and 
separate  enzymes  for  acting  upon  proteins,  fats  and 
carbohydrates  provided. 

Thus  the  digestive  function  ceases  to  be  a  cellular 
function  though  it  continues  to  be  the  result  of  the 
combined  activity  of  many  variously  specialized  cells. 

The  digestive  organs  also  gradually  come  to  stand  in 
close  relation  with  the  organs  of  excretion  so  that  offen- 
sive digestive  products  may  be  removed  from  the  blood 
before  it  is  distributed  to  the  general  system.  For  this 
purpose  the  liver  is  interposed  between  the  digestive 
organs  and  the  systemic  circulation. 

Vegetables  that  nourish  themselves  upon  compara- 
tively simple  inorganic  compounds  diffused  through  the . 
air   and  water   need   no   organs   of  digestion  while  to 
animals  that  nourish  themselves  upon  highly  complex 
vegetable  and  animal  proteins  they  are  indispensable. 

The  digestive  apparatus  thus  becomes  a  factor  of 
much  importance  in  animal  morphology  and  develop- 
ment. Except  among  the  protozoa  and  a  few  para- 
sitic worms  its  presence  is  invariable.  It  must  be 
proportioned  to  the  requirements  of  the  animal,  but 
just  as  the  animal  cannot  live  without  it,  it  cannot  be 
of  use  unless  means  are  furnished  for  providing  it  with 
material  to  work  upon.  Such  material  is  usually  ac- 
quired through  movements  effected  by  special  organs. 

The  Motor  System. — Here  we  have  to  consider  those 
organs  whose  primary  purpose  seems  to  be  to  enable  the 
organism  to  secure  its  food.  The  vegetable  world  is  with 
few  exceptions  without  organs  of  prehension,  motion, 
or  locomotion  because  they  are  not  needed.  Moisture 
sucked  from  the  soil  by  roots,  and  gases  and  moisture 
taken  from  the  air  by  leaves  constitute  the  materials 
upon  which  the  vegetable  world  subsists,  and  as  these 
are  always  available  no  special  organs  are  required  to 
obtain  them. 

.     Animal  organisms,  however,  must  have  food  in  the 
form  of  highly  combined  products,  only  to  be  derived 


12G 


BIOLOGY:  GENERAL  AND  MEDICAL 


—  nu. 


from  antecedent  forms  of 
life,  and  as  these  are  not 
to  be  found  everywhere, 
either  the  animal  must 
wait  until  such  come  to  it 
and  then  seize  them,  or  go 
in  search  of  them.  Thus 
comes  about  the  necessity 
which  is  met  by  the  de- 
velopment of  organs  of 
[—  c  motion,  locomotion,  and  pre- 
hension. 

The  unicellular  organ- 
isms show  the  most  primi- 
tive of  these  in  the  pseudo- 
pods  of  the  amreba,  and 
the  cilia  and  flagella  of 
the  infusoria.  Pseudopodia 
subserve  all  three  purposes, 
motion,  locomotion,  and 
prehension,  but  cilia  and 
flagella  are  higher  special- 
izations and  confine  their 
usefulness  to  motion,  by 
which  stationary  cells  pro- 
duce currents  in  the  sur- 
rounding fluids,  and  loco- 
motion by  which  the  cell 
is  propelled  through  the 
fluid  in  which  it  lives. 

Further  specializations 
also  occur  in  regard  to  the 

££3ESJfcSSi.fcJS3  «1!a'  <-ertain  of  the»  bdn« 

organ  is  developed;  en,  the  cnidocil  or      adapted  to  locomotion,  and 

"trigger";  cp,  the  capsule  or  nettling    certain    arranged    in   such 

organ;  /,  the  nettling  filament  or  lasso;  ]• 

».  neck   of    the    capsule;   nu.   nucleus      Banner    as     to     direct    CU1'- 

oftheceiL  rents  of   fluid  toward  the 

oral  orifice  of  the  organism. 


FIG.   43.— Nettling  cells  of  Hydra. 
(After  Schmeil.) 


THE    HIGHER    ORGANISMS 


127 


The  elementary  composits  com- 
posed of  fairly  homogeneous  ele- 
ments possess  no  special  motor  or- 
gans. Their  cells  may  be  amoe- 
boid as  in  microgromia,  or  they 
may  be  ciliated  as  in  volvox,  the 
cilia  being  so  disposed  as  to  serve 
the  best  interests  of  the  colony. 
In  volvox  they  are  placed  extern- 
ally to  permit  movement;  in  the 
porifera,  which  are  immobile,  the 
ciliated  cells  are  internally  disposed 
so  that  their  lashing  produces  cur- 
rents of  water  which  constantly 
flow  through  the  radiating  canals 
carrying  in  the  minute  particles 
upon  which  the  amoeboid  cells  of  -*  v~ 
the  entoderm  seize. 

True  prehensile  organs,  composed 
of  many  cells,  first  make  their  ap- 
pearance among  the  ccelenterates, 
and  are  most  simple  in  hydra, 
where  they  form  a  circle  of  from 
six  to  ten  long  slender  arms  about 
the  oral  aperture.  Each  of  these 
arms  or  tentacles  has  a  structure 
corresponding  with  the  body  of  the 
animal  itself,  that  is,  it  consists 


FIG.  44. — Enlarged  view  of  the  anterior  and 
posterior  parts  of  the  body  of  an  earthworm  as 
seen  from  the  ventral  aspect,  an,  Anus;  c,  clitel- 
lum;  g.p.,  glandular  prominences  on  the  twenty- 
sixth  somite;  in,  mouth;  o.d,  external  openings 
of  the  oviducts;  p.s,  prostomium;  s,  setae;  s.r, 
openings  of  the  seminal  receptacles;  s.d,  external 
openings  of  the  sperm  ducts.  The  form  of  the 
body  varies  greatly  in  life  according  to  the  state 
of  expansion.  The  specimen  here  shown  is  from 
•an  alcoholic  preparation  (slightly  enlarged).  (Sedg- 
wick  and  Wilson.) 


128  BIOLOGY:  GENERAL  AND  MEDICAL 

of  ectodermal  and  entodermal  cellular  layers,  and  is 
hollow,  the  space  communicating  with  the  general 
body  cavity.  The  ectodermal  cells  of  the  tentacles, 
however,  present  a  peculiar  specialization  not  seen  in 
other  parts  of  the  ectoderm,  namely,  the  possession  of 
certain  "nettling  cells,"  which  are  intended  to  aid  in 
securing  the  microscopic  organisms,  upon  which  the 
animal  preys,  by  stinging  or  stunning  them  in  order  that 
they  may  be  better  grasped  and  introduced  into  the 
body  cavity.  Each  of  these  nettling  cells  contains  a 
beautiful  mechanism  consisting  of  a  capsule  in  which  a 
long  stinging  filament  is  closely  coiled.  A  second 
small  filament,  trigger  or  cnidodl,  projects  from  the 
cytoplasm.  When  the  trigger  contacts  with  a  suitable 
object,  the  trap  springs  and  the  filament  is  suddenly 
thrown  out  against  it  with  stinging  and  paralyzing 
effect.  In  addition  to  the  nettling  cells  the  ectoderm 
seems  to  contain  certain  primitive  muscle  cells  which 
increase  the  mobility  of  the  tentacles. 

Though  the  tentacles  are  primarily  organs  of  pre- 
hension they  also  serve  as  organs  of  locomotion;  for, 
should  a  change  of  position  be  advantageous,  the  hydra 
bends  over,  seizes  an  object  with  its  tentacles,  lets  go  its 
foothold,  gradually  turns  over  and  effects  a  new  attach- 
ment elsewhere. 

Leaving  the  hydroids  and  passing  to  the  higher  coalen- 
terates  a  functional  differentiation  soon  separates  pre- 
hension, which  is  limited  to  the  tentacles,  from  locomo- 
tion which  is  effected  by  the  development  of  muscle 
cells  in  the  umbrella  as  in  medusa  where  the  alternate 
contraction  and  expansion  enables  the  animals  to  swim 
about. 

Distinct  locomotory  appendages  appear  in  the  seg- 
mented worms  in  the  form  of  bristles  or  setse  attached 
to  each  segment  and  directed  backward.  As  the  muscular 
movements  force  the  body  of  the  animal  forward  these 
appendages  catch  upon  irregularities  of  the  surface  upon 
which  it  moves,  and  prevent  retrogression  of  the  ad- 


THE   HIGHER   ORGANISMS  129 

vanced  segments  as  the  remainder  are  drawn  after  them. 

Among  the  arthropods  the  function  of  locomotion 
becomes  highly  specialized  by  the  development  of  jointed 
appendages  controlled  by  muscles  attached  to  all  or 
certain  of  the  segments. 

Among  the  vertebrates  the  same  general  plan  of 
having  the  motor  organs  spring  from  certain  of  the 
body  segments  is  preserved,  though  they  undergo  great 
modification  in  their  specialization  into  fins,  wings,  legs, 
arms,  nippers,  etc.,  with  complicated  muscular  and 
other  adjustments. 

Frequent  allusion  has  been  made  to  muscle  cells  and 
muscles  so  that  it  becomes  necessary  to  say  a  few  words 
about  these  as  important  adj  uncts  to  the  motor  apparatus. 

In  the  tentacles  of  certain  hydras,  in  the  higher  coelen- 
terates,  and  in  all  of  the  higher  animals  there  are  certain 
mesenchymal  cells  that  specialize  in  contractility. 
These  are  known  as  muscle  cells.  They  are  of  elongate 
shape,  but  are  capable  of  manifesting  their  contractile 
power  by  shortening  and  thus  making  traction  upon  the 
structures  to  which  they  are  attached.  Primarily  of 
this  spindle  shape  and  appearing  singly,  they  are  asso- 
ciated in  groups  and  bundles  in  the  higher  animals  where 
their  combined  action  is  very  effective  as  sources  of 
movement.  Eventually  they  appear  as  elongate  multi- 
nucleated,  transversely  striated  fibrils,  singly  or  in  bun- 
dles— the  voluntary  muscles — which  are  the  source  of 
the  extensive  and  powerful  movements  of  the  higher 
animals. 

The  Circulatory  System. — So  soon  as  cell  combinations 
become  so  large  or  so  differentiated  as  to  make  it  im- 
possible for  each  cell  to  exist  under  conditions  common 
to  all  the  cells,  it  becomes  desirable  that  some  special 
means  be  provided  by  which  the  less  advantageously 
situated  cells  may  be  provided  with  nourishment  and 
have  their  effete  products  removed. 

In  the  most  simple  cell  colonies,  such  as  Microgromia, 
Carchesium,  Epistylus,  and  Volvox,  the  cells,  though 


130  BIOLOGY:  GENERAL  AND  MEDICAL 

connected,  are  too  independent  to  feel  this  need;  but 
in  the  sponges  and  hydras  the  differentiation  of  the  cells 
becomes  sufficient  to  give  the  entoderraal  cells  an  advan- 
tage over  others  unless  some  means  for  transporting 
the  products  of  digestion  can  be  found.  In  all  prob- 
ability the  primitive  means  is  similar  to  that  seen  among 
plants  where  material  of  various  kinds  is  passed  directly 
from  cell  to  cell.  Such  primitive  methods  cannot  suffice, 
however,  except  in  cases  in  which  the  cell  groups  are 
small. 


FIG.  45.  — A  medusa  of  Obelia.     Seen  from  the  oral  surface,  magnified  (ad  not.) 
(After  Master-man.) 

Plants  soon  outgrow  the  direct  transfer  and  provide 
themselves  with  "vascular  tubules"  through  which  the 
sap  flows  in  a  continuous  current  from  the  roots  to  supply 
the  evaporation  in  the  leaves. 

The  lower  ccelenterates  among  animals,  by  contracting 
the  body,  force  its  contained  fluid  rich  in  nourishment 
into  every  part  including  the  hollow  tentacles. 

Ascending  a  little  higher  among  the  hydroids  we  find 
some  of  them — Coryne,  Obelia — giving  off  budding  off- 
spring minute  in  size  but  resembling  a  medusa  or  jelly 
fish  in  shape.  From  the  centre  of  each  of  these  embryos 


THE    HIGHER    ORGANISMS  131 

there  hangs  down  a  hollow  open  sac  called  the  manubrium. 
This  is  in  reality  its  stomach,  but  it  opens  into  several 
canals  that  radiate  from  the  centre  and  communicate 
with  another  canal  that  courses  along  the  margin  of  the 
disc.  This  slightly  more  complex  arrangement  is  known 
as  the  water  vascular  system.  It  begins  in  the  stomach 
and  from  it  carries  the  contents — water  with  products 
of  digestion — through  the  tubes  and  back  again,  thus 
affording  cells  remote  from  the  actual  organ  of  digestion 
an  opportunity  to  effect  an  exchange  of  useful  for  useless 
matter. 

As  we  ascend  to  the  true  jelly  fishes  we  find  the  same 


* 

Fia.  46. — Diagrammatic  sagittal  section  of  Microstomum,  showing  a  chain  of 
four  zooids  produced  by  fission,  b,  Brain  of  the  original  zooid  (the  exponents 
indicating  corresponding  structures  of  the  more  recently  formed  zooids);  c, 
ciliated  pit;  d,  dissepiments  indicating  different  stages  in  the  separation  of  the 
zooids;  e,  eyespot;  ent,  entoderm:  0,~gut;  gl,  glandular  cells  about  the  mouth; 
m,  mouth  of  the  original  worm.  (Galloway.) 

arrangement,  the  only  difference  being  in  the  number  of 
radiating  tubes  and  an  increasing  complexity  of  anasto- 
mosing branches  by  which  the  .circulating  fluids  are 
permitted  to  come  in  contact  with  a  greater  number  of 
cells.  The  propulsive  force  is  found  solely  in  the  mus- 
cular movements  made  by  the  swimming  animal  as  it 
opens  and  closes  its  transparent  umbrella. 

Among  the  unsegmented  worms  the  device  for  dis- 
tributing nourishment  does  not  differ  fundamentally 
from  what  has  already  been  described.  It  consists  of  a 
water  vascular  system  comprising  two  main  lateral 
tubes  with  many  branches  extending  to  the  periphery 


132 


BIOLOGY:  GENERAL  AND  MEDICAL 


of  the  body.  A  new  specialization  makes  its  appearance, 
however,  for  at  least  a  part  of  the  fluid  thus  distributed 
does  not  return  again  to  the  stomach-intestine,  but 
leaves  the  body  through  pores  guarded  by  special  cells. 
Here  in  intimate  relation  to  the  primitive  circulatory 
apparatus  we  find  the  inception  of  the  essential  function 
of  excretion  or  the  elimination  of  effete  matter. 


FIG.  47.  — Diagram  to  indicate  the  course  of  the  blood  in  the  nymph  of  a 
dragon  fly.  Epitheca.  a,  Aorta;  h,  heart;  the  arrows  show  directions  taken  by 
currents  of  blood.  (After  Kolbc.} 

The  crudity  of  a  system  that  permits  the  entire  con- 
tent's of  the  alimentary  canal  to  enter  the  circulating 
pabulum  is  superseded  by  complete  separation  of  the 
digestive  and  circulatory  systems;  with  corresponding 
improvement  in  the  quality  of  the  systems  thus  sepa- 
rated, the  blood  can  no  longer  be  propelled  by  move- 
ments of  the  alimentary  apparatus  and  for  its  proper 
circulation  it  becomes  essential  that  the  great  vessels 


THE    HIGHER   ORGANISMS  133 

be  contractile,  a  specialization  readily  observed  among 
the  lower  worms  and  laying  the  foundation  of  the  organ 
known  as  the  heart. 

The  larger  vessels  are  at  first  in  intimate  relation  with 
the  alimentary  tract  which  they  surround  with  loops. 
Muscular  fibres  are  present  in  these  large  vessels  so 
that  as  the  products  of  digestion  are  absorbed  into  the 
blood,  a  slow  rhythmical  contraction  propels  the  blood 
in  a  circuit  of  the  tissues.  At  first  the  arrangements  are 
so  primitive  that  the  course  of  the  blood  is  uncertain, 
but  as  the  specialization  becomes  improved  there  is  an 
increasing  tendency  for  the  flow  to  maintain  a  constant 
direction,  efferent  and  afferent  vessels  being  differen- 
tiated and  a  primitive  separation  of  arteries  and  veins 
thus  established.  In  the  elementary  form  in  which 
this  condition  is  observed  there  are  no  capillary  vessels 
connecting  the  two  so  that  the  circulation  is  not  closed. 
True  capillaries  first  appear  among  certain  of  the  worms, 
though  many  higher  animals — as,  for  example,  insects — 
are  without  them.  In  the  higher  annulates  and  among 
the  arthropods  the  major  vessel  becomes  expanded  into 
a  primitive  heart  which  receives  the  blood  from  several 
large  veins  whose  orifices  are  provided  with  valves  pre- 
venting backward  flow  so  that  the  stability  of  the  circula- 
tion is  established.  The  muscular  movements  of  the 
heart  now  become  rhythmical,  regular,  and  slow.  Such 
simple  hearts  are  found  among  the  arthropoda  generally. 

The  animals  thus  far  used  to  exemplify  the  increasing 
complexity  of  the  circulatory  system  are  aquatic,  of  small 
size,  and  of  simple  structure.  But  as  the  phylogenetic 
series  is  ascended,  and  increase  in  complexity  is  brought 
about  through  the  differentiation  of  systems  of  organs, 
increase  in  size,  and,  eventually,  change  from  the  aquatic 
to  the  terrestrial  mode  of  life,  a  new  requirement  neces- 
sitates the  development  of  a  new  system  of  organs  as 
well  as  a  further  increase  in  the  complexity  of  the  al- 
ready existing  organs.  This  is  an  increase  in  the  O  sup- 
ply and  an  improvement  in  the  means  by  which  it  is  re- 
ceived and  distributed,  and  is  met  by  the  appearance  of 


134  BIOLOGY:  GENERAL  AND  MEDICAL 

the  respiratory  system.  The  relatively  simple  organisms 
absorb  O  from  the  fluids  in  which  they  live,  at  first  by 
surface  absorption,  then,  when  differentiated  into  an  outer 
derm  and  an  inner  gastric  cavity,  partly  by  absorption 
from  the  external  surface  and  partly  through  the  gastric 
contents,  then  by  the  transmission  of  the  constantly 
changing  gastric  contents  through  the  gastro-vascular 
system.  When  the  blood  becomes  a  permanently  differ- 
entiated fluid  enclosed  in  vessels,  some  oxygenation  is 
effected  through  the  surface  of  the  body  as  the  blood  is 
slowly  moved  about  by  the  primitive  heart,  but  as  the 
complexity  of  the  organisms  increases  and  large  groups 
of  cells  are  set  aside  for  various  definite  purposes,  the 
supply  of  oxygen  thus  secured  becomes  inadequate  for  the 
support  of  the  tissues,  and  it  becomes  necessary  that  special 
oxygen-absorbing  organs  be  provided  and  that  the  blood 
be  regularly  brought  to  them.  This  necessitates  an  im- 
provement in  the  blood  itself,  by  which  oxygen  absorption 
may  be  increased,  and  an  improvement  in  the  means  of 
circulating  it  in  order  that  the  freshly  oxygenated  blood 
may  not  be  free  to  mix  with  that  whose  oxygen  has  already 
been  exhausted — that  is,  a  separation  of  arterial  and 
venous  blood. 

As  has  been  shown,  the  pabulum  supplied  to  the  cells  of 
the  most  lowly  forms  of  life  differs  from  the  surrounding 
fluid  in  which  the  animal  lives  only  in  containing  an 
increased  quantity  of  nutritious  material  available  for 
absorption  or  direct  incorporation  by  the  cells,  this 
condition  persisting  until  the  separation  of  the  vascular 
system  from  the  digestive  system  is  complete.  The 
nutrient  pabulum  then  first  deserves  the  name  blood. 
It  continues  for  some  time  to  be  an  aqueous  fluid. 
Occasionally  one  finds  a  few  amceboid  cells  from  the 
mesenchyme  circulating  in  it  and  picking  up  any  solid 
particles  that  may  accidentally  enter.  As  the  scale  of 
life  is  ascended  the  number  of  these  increases  and  their 
occurrence  becomes  more  regular  until  in  molluscs  and 
arthropods  these  amceboid  "white  corpuscles"  are  con- 
stant elements  of  the  blood.  The  blood,  in  the  mean- 


THE    HIGHER    ORGANISMS  135 

time,  becomes  more  concentrated  and  as  its  specific 
gravity  increases  its  oxygen-absorbing  capacity  is  also 
increased. 

With  the  appearance  of  true  respiratory  organs — 
gills  in  the  Crustacea — the  circulation  becomes  modified 
in  a  simple  fashion.  The  primitive  heart  discharges 
the  blood  into  several  large  vessels  one  of  which  conveys 
it  principally  to  the  gills,  where  it  is  aerated  as  will  be 
shown  later,  from  which  it  returns  to  the  heart  to  become 
mixed  with  the  blood  returning  through  other  afferent 
vessels,  imparting  its  oxygen  to  the  whole  with  which 
it  mixes  before  being  sent  upon  a  new  circuit.  The 
aerating  circuit  is  not  definitely  separated  from  that  of 
the  tissues  in  the  neighborhood  of  the  gills;  a  very  small 
fraction  of  the  total  blood  is  carried  to  the  gills  and 
after  being  aerated  it  mixes  with  the  general  blood  mass. 
The  arrangement  is  very  imperfect  when  contrasted  with 
that  found  in  mammals,  but  answers  the  necessities  of 
the  animals  in  which  it  occurs. 

Among  some  of  the  invertebrates  the  blood  corpuscles 
are  found  to  contain  small  quantities  of  a  reddish  sub- 
stance known  as  hemoglobin  which  forms  a  loose  combi- 
nation with  oxygen  highly  advantageous  to  the  blood 
by  increasing  its  oxygen-carrying  power.  Among  the 
vertebrates,  however,  the  blood  corpuscles  are  always 
of  two  kinds,  the  whites  or  leucocytes  which  are  amoeboid 
and  contain  no  hemoglobin,  and  the  reds  or  erythrocytes 
which  invariably  contain  it,  the  proportion  of  the  latter 
increasing  until  among  the  highest  mammals  they  exceed 
the  leucocytes  in  the  proportion  of  1  white  to  750  red. 
At  first  the  corpuscles  scarcely  differ  from  one  another 
except  in  containing  hemoglobin,  but  eventually  the 
erythrocytes  become  so  differentiated  that  they  appear 
only  as  minute  discs  or  cups  of  hyaline  stroma  without 
nuclei  and  thoroughly  impregnated  with  hemoglobin. 
This  enables  the  blood  to  absorb  and  transport  to  the 
tissues  immensely  more  oxygen  than  would  otherwise 
be  possible. 


136 


BIOLOGY:  GENERAL  AND  MEDICAL 


As  the  perfection  of  the  oxygen  absorbing  and  dis- 
tributing quality  of  the  blood  thus  improves,  the  means 
of  circulating  it  also  improves  through  more  complex 
adjustments  of  the  heart  and  vascular  system  by  which 
freshly  aerated  blood  is  continually  supplied  to  the  tissues 
and  organs  and  is  prevented  from  mixing  with  the  ex- 


c.  v.  r.. 


FIG.  48. — Diagram  of  the  heart,  the  branchial  arches,  and  the  principal  veins 
in  the  Teleosts.  Ventral  view.  The  heart  is  represented  without  the  sigrnoid 
flexure;  that  is,  with  the  auricle  posterior,  a,  Aorta;  aw,  auricle;  br.a,  branchial 
arches  of  the  aorta  (1-4,  numbering  from  the  front);  c,  carotid;  c.v,  cardinal 
veins  (right  and  left);  d.a,  dorsal  arteries;  /,  jugular  veins;  d.c,  ductus  Cuvieri; 
s.v.,  sinus  venosus;  v,  ventricle.  (Galloway.) 

hausted  blood  returning  from  them.  Thus  there  come 
to  exist  two  distinct  circulations:  one  for  the  aeration  of 
the  blood,  the  other  for  the  nutrition  of  the  organs 
and  tissues. 

The  transformation  in  the  structure  of  the  heart  by 
which  this  is  made  possible  is  not  difficult  to  understand. 


THE    HIGHER    ORGANISMS  137 

In  the  fishes  the  heart  is  tubular  and  consists  of  two 
chambers,  a  posterior  auricle  and  an  anterior  ventricle. 
As  in  the  Crustacea,  the  blood  is  forced  by  the  anteriorly 
situated  ventricle  into  the  aorta,  which  gives  off  large 
branchial  arteries  in  pairs,  itself  dividing  to  form  the 
anterior  pair,  passes  through  the  branchial  arteries  to 
the  gills  where  it  is  aerated,  and  is  then  collected,  beyond 
the  gills,  into  two  dorsal  arteries,  by  which  it  is  distributed 
throughout  the  body  of  the  fish.  After  passing  through 
the  capillaries,  it  is  collected  by  two  large  cardinal  veins, 
from  which  it  is  brought  through  two  vessels — ducti 
cuvieri — into  the  sinus  venosus,  passed  into  the  auricle, 
and  then  into  the  ventricle  to  renew  the  circuit. 

It  is  interesting  to  find  three  genera  of  fishes,  survivors 
of  forms  common  in  past  geological  periods,  which 
occupy  a  position  intermediate  between  fishes  and 
batrachia  in  so  far  as  their  circulatory  apparatus  are 
concerned.  Of  these  Ceratodus,  the  Australian  "lung 
fish,"  is  more  like  other  fishes,  while  Protopterus  and 
Lepidosiren,  the  African  "mud  fish,"  are  more  like  the 
batrachians.  All  are  peculiar  in  possessing  lungs  as 
well  as  gills,  the  former  a  single  lung,  the  latter  a  pair  of 
lungs,  and  in  having  their  circulatory  apparatus  modi- 
fied in  consequence. 

In  Ceratodus  there  is  one  lung  which  is  small  and  of 
far  less  value  as  an  aerating  organ  than  the  gills.  Indeed 
the  quantity  of  blood  that  is  carried  to  it  is  very  small, 
and  has  already  passed  through  the  gills,  so  as  not  to  re- 
quire this  supplementary  aerating  action  except  when 
the  fish  is  prevented  from  using  its  gills,  during  periods 
of  drought  when  the  ponds  dry  up  or  the  water  they  con- 
tain becomes  thick  and  muddy,  through  evaporation, 
and  charged  with  offensive  substances  and  fermentative" 
gases.  It  is  then  that  the  lung  subserves  a  useful  pur- 
pose by  tiding  the  fish  over  what  might  be  called  a  period 
of  air  famine,  and  permitting  the  blood  to  come  into 
contact  with  just  enough  air  to  enable  life  to  be  main- 
tained. This  extremely  primitive  pulmonary  develop- 


138 


BIOLOGY:  GENERAL  AND  MEDICAL 


ment  is  unconnected  with  important    changes  in  the 
heart  or  great  vessels. 

In  Protopterus  and  Lepidosiren,  however,  the  fishes 
are  not  only  provided  with  gills,  but  also  with  two  lungs 
of  considerably  larger  size  and  of  greater  importance. 


c.  v.  I. 


p.c. 


FIG.  49. — Diagram  of  the  heart  and  branchial  arches  in  Ceratodus  (one  of  the 
Dipnoi).  Position  and  lettering  as  in  the  preceding  cut.  ab.  air  bladder  (lung) ; 
p. a,  pulmonary  artery;  p.c,  postcaval  vein  (right);  p. v,  pulmonary  vein.  (Gallo- 
way.) 

In  considering  the  changes  necessitated  through  this 
improvement  we  must,  however,  bear  in  mind  that  the 
fishes  by  preference  and  under  all  favorable  conditions, 
continue  to  live  the  life  of  fishes,  remaining  under  water, 
and  aerating  their  blood  by  means  of  gills  and  that  it  is 
only  under  exceptional  circumstances  that  the  use  of 


THE    HIGHER    ORGANISMS 


130 


lungs  is  demanded.  For  this  reason  the  gills  continue  to 
form  the  chief  aerating  organs,  and  the  lungs  an  auxiliary 
mechanism  to  be  held  in  reserve,  hence  .the  circulatory 
arrangements  continue  more  closely  to  resemble  those 
of  gill-breathers  than  those  of  lung  breathers.  The  bulk 
of  the  blood  leaving  the  ventricle  passes  into  an  aorta, 


post 


at?. 


at). 


(La. 


FIG.  50. — Diagram  of  the  heart  and  branchial  arches  in  Protopterua  (one  of 
the  Dipnoi).  Position  and  lettering  as  in  the  preceding,  pre.c,  precaval  vein, 
made  up  of  right  and  left  jugulars,  subclavians,  etc.;  post.c,  postcaval,  made  up 
of  the  cardinals,  right  and  left.  (Galloway.) 

then  through  the  branchial  arteries  and  is  systemically 
distributed,  while  a  small  portion  passes  to  the  lungs, 
and  then  through  pulmonary  veins  into  the  auricle. 
The  only  modification  of  the  heart  itself  is  a  partial 
division  of  the  auricle  into  two  ill-defined  chambers — 
right  and  left  auricles.  The  right  auricle  which  receives 


140 


BIOLOGY:  GENERAL  AND  MEDICAL 


the  systemic  blood  is  much  the  larger  of  the  two.  In 
both  these  lung-breathing  fishes  the  blood  undergoes  a 
partial  separation  for  that  which  goes  to  the  lungs  returns 
to  the  heart  before  it  makes  the  systemic  circuit.  So 
that  when  the  lungs  are  functional  and  the  gills  inactive 
the  blood  returning  from  the  systemic  circuit  is  in  part 


FIG.  .51. — Diagram  of  the  heart  and  branchial  arches  in  the  Frog.  c.g, 
carotid  gland;  I,  lungs;  La,  left  auricle;  r.a,  right  auricle.  (Galloway.) 

passed  through  the  lungs  before  being  again  returned 
to  the  systemic  circuit. 

Among  the  batrachians  the  separation  of  the  auricles 
becomes  distinct.  With  a  very  few  exceptions,  these 
animals  are  gill-breathers  in  the  larval  stage,  and  lung 
breathers  in  adult  life.  This  change  necessitates  a 
transformation  of  the  vascular  arrangements  as  the 


THE    HIGHER    ORGANISMS 


141 


aquatic  is  abandoned  for  the  terrestrial  mode  of  life. 
During  embryonal  life  the  circulation  is  carried  on 
much  as  it  is  in  the  fishes,  the  branchial  arches  being 
conspicuous,  but  upon  the  attainment  of  adult  life,  and 
the  establishment  of  a  pulmonary  circulation  the  bran- 


Mr.c 


FIG.  52. — Diagram  of  the  heart  and  branchial  arches  in  a  reptile.  Position 
and  lettering  as  in  preceding  figures,  l.v,  left  ventricle;  r.v,  right  ventricle 
(Galloway.) 

chial  arteries  atrophy  and  true  pulmonary  circulation 
takes  its  place. 

The  frog  affords  an  excellent  example  of  the  batrachian 
type  of  circulation.  The  heart  is  distinctly  three-cham- 
bered, having  one  ventricle  and  two  completely  separated 
auricles.  The  blood  discharged  by  the  ventricle  passes 


142 


BIOLOGY:  GENERAL  AND  MEDICAL 


to  both  systemic  and  pulmonary  systems  of  vessels,  that 
of  the  systemic  circulation  returning  to  the  right  auricle, 
that  from  the  pulmonary  to  the  left.  Should  both 
of  these  auricles  discharge  the  blood  into  the  ventricle 
without  some  provision  for  separating  their  contents, 


Fio.  53. — Diagram  of  the  heart  and  the  branchial 
dotted  outline  of  the  arches  of  the  fish  is  drawn  for 
auricles  are  represented  in  a  posterior  position,  us  i 
(Galloway.) 


rchra  in  mammals.  A 
jady  comparison.  The 
the  preceding  figures. 


much  of  the  advantage  gained  by  the  auricular  separa- 
tion would  be  lost.  The  substance  of  the  ventricle  is, 
however,  peculiar  in  its  spongiriess,  so  that  as  the  blood 
enters  the  freshly  aerated  portion  from  the  left  auricle 
is  kept  apart  from  the  exhausted  blood  of  the  right 
ventricle,  and  when  the  ventricular  contraction  takes 


THE   HIGHER   ORGANISMS  143 

place  the  blood  is  discharged  in  such  manner  that  the 
freshly  aerated  portion  rushes  into  the  aorta  and  to  the 
head  and  brain  of  the  animal,  while  the  exhausted  blood 
later  follows  the  greater  part  of  it  going  to  the  lungs. 

Similar  primitive  arrangements  obtain  among  the 
batrachia,  generally  and  also  among  the  reptilia  with 
the  exception  of  the  crocodiles.  The  reptilian  heart  io 
somewhat  improved  over  that  of  the  batrachia,  but 
it  is  only  in  the  crocodiles  that  the  ventricle  becomes 
completely  divided  by  a  septum. 

In  birds  and  mammals  the  circulatory  system  attains 
perfection  in  the  sense  that  the  heart  is  completely  four- 
chambered  and  is  thus  able  to  effect  a  complete  separa- 
tion of  the  aerated  or  arterial  from  the  exhausted  or 
venous  blood.  In  these  animals  the  blood  leaving  the 
left  ventricle  is  distributed  by  the  aorta  and  its  branches 
to  the  entire  systemic  circulation  where  it  passes  through 
the  capillaries  and  is  gathered  together  in  two  large 
veins,  or  vence  cavce,  which  convey  it  into  the  right  auricle. 
From  the  right  auricle  it  passes  into  the  right  ventricle, 
and  thence  through  the  pulmonary  artery  to  the  lungs. 
Having  passed  through  the  pulmonary  capillary  plexuses 
for  aeration,  it  is  collected  in  several  pulmonary  veins 
by  which  it  is  returned  to  the  left  auricle,  from  which  it 
passes  into  the  left  ventricle  and  again  makes  the  sys- 
temic circuit.  By  these  means  a  certain  quantity  of 
freshly  aerated  blood  is  constantly  being  distributed  to 
the  viscera  for  the  support  of  their  cells,  while  an  equal 
quantity  is  always  being  sent  to  the  lungs  to  be  freshly 
aerated.  The  two  circulations  being  independent  of 
one  another,  no  opportunity  is  ever  afforded  for  the 
venous  and  arterial  bloods  to  mix. 

The  Respiratory  System. — Respiration  being  an  indis- 
pensable function  of  living  substance  early  requires 
special  means  by  which  it  shall  be  made  possible  for 
all  the  cells  of  the  composite  animal  to  receive  oxygen. 

The  unicellular  organisms  whose  activities  and  require- 
ments typify  those  of  the  cells  of  the  higher  composite 


144  BIOLOGY:  GENERAL  AND  MEDICAL 

organisms  absorb  their  supply  of  oxygen  from  the 
medium  in  which  they  live.  This  is,  in  fact,  exactly  what 
the  cells  of  the  higher  organisms  do,  except  that  the 
medium  in  the  first  case  is  the  water  or  atmosphere  in 
which  the  cells  live  and  in  the  latter  the  blood  that  is 
distributed  to  them. 

Among  the  lower  forms  of  life  nutrition  and  oxygena- 
tion  are  intimately  associated,  and  it  is  not  until  consider- 
able complexity  of  structure  is  attained  that  it  becomes 
necessary  to  provide  special  organs  for  the  purpose  of 
aerating  the  blood. 

Thus,  in  the  porifera  or  sponges,  the  ciliated  cells  of 
the  entoderm,  by  causing  currents  of  water  to  flow  con- 
stantly through  the  various  body  pores,  keep  the  cells 
of  the  animal  constantly  aerated.  In  hydra  the  cells  of 
the  ectoderm  probably  absorb  oxygen  from  the  water 
surrounding  them,  while  those  of  the  entoderm  absorb 
it  from  the  water  in  the  body  cavity.  In  the  higher 
ccelenterates  with  a  primitive  vascular  system,  the  circu- 
lating nutritious  water  distributed  to  the  cells  conveys 
sufficient  oxygen  to  support  such  cells  as  may  not  be 
able  to  secure  it  from  the  surface  of  the  body. 

Among  the  unsegmented  worms  where  the  water 
vascular  system  is  improved,  respiration  is  still  carried 
on  partly  through  the  surface  of  the  body  and  partly 
by  the  primitive  blood,  but  as  the  structural  improve- 
ment confines  the  blood  in  vessels,  or  at  least  completely 
separates  it  from  the  contents  of  the  digestive  tube,  some 
adaptation  must  be  provided  for  supplying  oxygen  to 
the  blood  of  the  animal. 

In  their  most  simple  form  these  consist  of  slight  bulg- 
ings  or  projections  of  the  surface  corresponding  with 
thin  points  in  the  dermal  covering  of  the  animal,  at  which 
the  blood  more  easily  takes  up  O  and  discharges  its  CO2 
than  elsewhere.  Such  devices  constitute  the  primi- 
tive branchice  or  gills,  the  first  of  the  special  organs  of 
respiration. 

As  the  scale  of  anatomical  complexity  is  ascended  the 


THE    HIGHER   ORGANISMS  145 

size,  number,  distribution,  structure,  and  arrangement 
of  the  branchiae  undergo  great  modification,  but  they 
continue  to  be  the  only  means  of  effecting  aeration  of  the 
blood  so  long  as  aquatic  life  continued.  Modified 
branchiae,  indeed,  persist  among  terrestrial  mollusks. 

In  general  arrangement  the  branchiae  consist  of  more 
or  less  well-protected,  simple  or  complex  surfaces  upon 
which  the  blood  of  the  animal  is  brought  to  the  surface 
of  the  body,  and  in  intimate  contact  with  the  surround- 
ing water  in  order  that  the  exchange  of  gases  may  be 
effected. 

So  soon  as  terrestrial  life  is  adopted  lungs  are  de- 
veloped, and  the  atmosphere  rich  in  oxygen  is  taken  into 
the  body  and  there  aerates  the  blood.  Lungs  at  first  ap- 
pear as  relatively  simple  sacs  into  which  air  is  drawn, 
and  in  the  walls  of  which  innumerable  capillaries  ramify. 
Soon,  however,  the  structure  becomes  more  and  more 
divided  into  minute  sacs  or  alveoli,  in  the  walls  of 
which  the  capillaries  ramify  so  that  the  amount  of  aerat- 
ing surface  is  enormously  increased  and  the  gaseous 
exchange  made  correspondingly  easy. 

With  the  development  of  the  lungs  special  means 
must  be  provided  for  creating  the  necessary  vacuum 
by  which  the  air  is  to  be  drawn  in.  In  the  lower  verte- 
brates (reptiles)  among  whom  the  breathing  is  slow 
and  not  very  regular  this  is  accomplished  by  the  com- 
bined movements  of  many  of  the  body  muscles,  but  in 
the  higher  vertebrates  the  body  cavity  is  divided  by  a 
transverse  muscular  partition,  known  as  the  diaphragm, 
whose  contractions  and  relaxations  are  the  chief  source 
of  the  respiratory  movements. 

The  Excretory  System. — Vital  activity,  being  a  chemi- 
cal process  effected  through  the  oxidation  of  protoplasm, 
is  inevitably  attended  with  the  formation  of  combus- 
tion products.  Of  these  the  organism  can  make  no 
further  use,  partly  because  their  molecular  structure 
is  more  stable  than  that  of  the  protoplasm  itself  and 
partly  because  the  energy  required  to  resynthesize  them 
10 


146  BIOLOGY:  GENERAL  AND  MEDICAL 

would  be  equal  to  the  whole  value  thus  gained.  The 
effete  matter  is,  therefore,  a  useless  encumbrance  to 
the  organism,  and  when  derived  from  nitrogenous  com- 
pounds is  injurious  if  retained,  so  that  we  find  even  the 
most  lowly  creatures  eliminating  or  throwing  off  waste 
products  in  some  form  or  other,  the  process  being  known 
as  excretion. 

As  nearly  all  of  the  lowly  forms  of  life  are  aquatic, 
their  excreta  are  easily  carried  away  by  transfusion. 
In  the  corticate  protozoa  they  may  be  suddenly  elim- 
inated through  an  anal  pore  in  the  ectoderm. 

Primitive  metazoan  animals  whose  cells  are  in  two 
layers,  one  outer  in  contact  with  the  water  of  their 
habitat,  the  other,  inner,  in  contact  with  the  water 
alternately  sucked  in  and  forced  out  of  the  gastric 
cavity,  as  in  hydra,  or  carried  through  in  a  continuous 
stream,  as  in  the  porifera,  need  no  special  contrivance 
for  the  removal  of  their  cellular  excrement  which  is 
transfused  or  ejected  from  the  cells. 

It  is,  therefore,  not  until  structural  complexity  em- 
bracing a  separation  of  the  blood  from  the  gastric  con- 
tents is  reached  and  the  cells  become  so  numerous  that 
many  of  them  are  remote  from  both  surface  water  and 
watery  gastric  contents,  that  some  special  contrivance 
by  which  the  cellular  waste  products  can  be  discharged 
is  required.  It  is  also  only  at  this  time  that  a  separation 
between  the  waste  that  results  from  indigestible  rem- 
nants of  food  in  the  gastric  cavity  and  the  waste  that 
results  from  cellular  metabolism  becomes  clear.  The 
former,  remaining  in  the  alimentary  organs,  is  discharged 
through  an  anal  orifice;  the  latter,  collected  by  the 
blood,  is  eliminated  through  certain  lateral  pores  along 
the  sides  of  the  animal's  body.  In  the  description  of 
the  circulation  of  the  unsegmented  worms  it  was  shown 
that  the  contents  of  the  water  vascular  system  in  part 
returns  to  the  digestive  organs,  but  that  a  small  part  of 
it  escapes  through  superficial  pores  of  the  skin.  This 
is  probably  the  most  primitive  form  in  which  excretion 


THE   HIGHER    ORGANISMS 


147 


appears  AS  a  distinct  function.  It  is  not,  however, 
quite  so  simple  a  function  as  would  appear  at  first 
sight,  for  upon  examination  it  is  found  that  t^e  fine 
branches  of  the  water  vascular  system  by  which  the  cir- 
culating pabulum  is  conveyed  to  the  surface  do  not 
terminate  in  simple  openings,  but  in  specialized  cells 
of  the  dermal  covering  of  the  animal,  known  as  "  flame- 
cells" — so-called  because  an  appearance  suggesting 
the  flickering  of  a  flame  is  caused  by  the  movement  of 
vibratile  cilia  situated  in  vacuoles  of  these  cells.  The 
tubules  terminate  in  vacuoles  of  these  cells  which  seem 


FIG.  54. — Diagram  of  a  nephridium  (simple  kidney  tubule)  of  a  segmented 
worm.  b,bl,  blood  vessels;  c,  coelome;  d,  duct  of  the  nephridium;  e,  external 
opening;  cf,  ciliated  funnel  opening  into  coelome;  gl,  glandular  or  secreting 
portion;  s,  septum;  W,  body  wall  composed  of  longitudinal  muscle  fibres, 
circular  fibres,  and  epithelial  layer;  w,  wall  of  gut.  (Galloway.) 

to  carry  on  the  excretory  function.  It  is  not  a  mere 
percolation  of  fluid  through  pores  with  which  we  have 
to  do,  but  a  function  of  certain  specialized  cells. 

In  the  annulata  we  find  special  organs  of  excretion, 
known  as  nephridia,  a  pair  of  which  is  found  in  each 
segment.  Each  nephridium  consists  of  a  much  con- 
voluted epithelial-lined  tubule  which  begins  in  a  funnel- 
shaped  ciliated  orifice  opening  into  the  ccelomic  cavity 
of  the  animal  and  directed  anteriorly.  This  gathers  up 
the  body  fluids  and  passes  them  through  the  convoluted 
portion  of  the  tubule  from  which  they  eventually  escape 


148  BIOLOGY:  GENERAL  AND  MEDICAL 

through  an  external  opening  or  pore.  The  blood  in  the 
vessels  circulates  through  a  vascular  plexus  about  the 
convoluted  portion  of  the  tubule  permitting  its  cells  to 
take  up  the  offensive  substances  and  transmit  them  to 
the  fluid  constantly  passing  through  the  lumen.  Here 
we  have  a  most  important  and  interesting  specialization 
and  differentiation  of  cellular  activity,  the  sole  function 
of  these  complicated  organs  being  the  absorption  of 
waste  products  from  the  blood  and  their  elimination 
from  the  animal. 

This  plan  of  having  a  tubular  gland  whose  epithelial 
cells  secrete  the  solids  which  are  carried  out  by  a  passing 
current  of  water  finds  no  improvement  as  the  scale  of 
zoological  life  is  ascended.  There  are  various  modifica- 
tions seen  among  special  groups  of  animals,  as  among  the 
Crustacea,  where  special  excretory  glands  are  situated 
near  the  mouth  parts  and  are  developed  upon  a  different 
principle,  but  in  the  main  the  only  difference  between 
the  nephridium  of  the  annelid  and  the  kidney  of  the  ver- 
tebrate is  to  be  found  in  the  number  of  component  ele- 
ments and  the  exact  means  by  which  the  watery  part  of 
the  excretion  is  provided. 

The  chief  excretory  organs  of  the  vertebrates  are 
known  as  kidneys,  which  upon  superficial  examination 
appear  highly  complex,  though  upon  investigation  are 
easily  resolvable  into  a  combination  of  units  each  of  which 
is  a  tubular  structure  whose  epithelial  cells  secrete  the 
solids  which  are  washed  away  by  the  water  supplied  by  a 
capillary  tuft  at  its  commencement.  Thus  each  struc- 
tural unit  in  the  mammalian  kidney  is  the  homologue  of 
the  nephridium  of  the  worm. 

Innervation  and  Coordination. — As  structural  differ- 
entiation and  specialization  increase,  and  that  cellular  in- 
dependence, by  virtue  of  which  any  cell  of  the  primitive 
composits  seems  able  to  perform  any  function,  gives  place 
to  organized  structure  in  which  certain  cells  are  set  aside 
for  the  performance  of  particular  functions,  means  of 
communication  between  the  different  cell  groups  becomes 


THE   HIGHER   ORGANISMS  149 

not  only  advantageous  but  indispensable.  The  more 
highly  specialized  a  cell  group  becomes,  the  more  isolated 
it  becomes,  and  the  more  imperative  becomes  the  neces- 
sity for  intercommunication,  control,  and  coordination. 

In  loosely  organized  cellular  combinations,  such  as 
Epistylus  and  Microgromia,  little  advantage  is  to  be 
gained  for  any  cell  from  impulses  derived  from  other 
cells,  though  the  general  irritability,  contractility,  and 
conductivity  of  the  protoplasm  may  enable  impulses  to 
pass  from  cell  to  cell. 

When  one  cell  of  a  simple  colony  is  disturbed  a  defen- 
sive reaction  is  sometimes  manifested  by  its  fellows,  and 
in  the  case  of  Carchesium  may  result  in  escape  from  danger 
through  the  contraction  of  the  stalk,  which  draws  away 
all  the  members  of  the  colony.  But  in  such  cases,  as  well 
as  in  the  case  of  the  sponges  and  the  hydras,  there  is 
usually  little  danger  to  cells  other  than  those  immediately 
menaced. 

If  a  portion  of  a  sponge  or  a  tentacle  of  a  hydra  be  bitten 
off  or  torn  away,  the  whole  organism  is  scarcely  affected 
and  the  damage  is  soon  repaired  by  regeneration. 

Among  plants  the  specialization  of  structure  rarely 
reaches  a  point  at  which  injury  to  a  part  seriously  affects 
the  whole,  which  fact,  added  to  the  immobility  of  plants 
in  general,  has  determined  that  their  entire  evolution  has 
taken  place  without  the  development  of  any  recognizable 
regulating  or  communicating  system  making  its  appear- 
ance even  in  the  highest  forms. 

But  in  animals  that  do  move  about  in  search  of  the 
particular  food  upon  which  they  live,  and  finding  it,  must 
apprehend  it,  comminute  it,  ingest  it,  digest  it,  circulate 
it,  and  assimilate  it,  directing,  regulating,  communicating, 
and  coordinating  mechanisms  become  essential,  and  are 
developed  in  the  form  of  the  nervous  system,  through  which 
impulses  received  from  without  enable  the  animal  to  adapt 
itself  to  the  conditions  of  its  environment — fly  from  danger, 
pursue  its  food,  etc. — and  impulses  received  from  within 
enable  it  to  adapt  itself  to  internal  conditions  so  that  its 


150  BIOLOGY:  GENERAL  AND  MEDICAL 

functions  may  be  carried  on  regularly  and  economically — 
digestive  enzymes  be  passed  into  the  digestive  organs  only 
when  there  is  something  to  digest,  etc. 

As  has  been  pointed  out  by  Loeb,  we  are  unable  to 
find  any  essential  difference  between  nerve  tissue  and 
other  tissue  except  that  it  possesses  greater  irritability 
and  conductivity.  It  goes  without  saying,  however,  that 
these  particular  qualities  must  be  so  adjusted  as  to  fulfil 
the  requirements  expressed  above. 

In  its  most  primitive  form,  with  beginning  differentia- 
tion, the  nervous  tissue  is  scarcely  to  be  recognized.  In 
the  ectodermal  cells  of  hydra,  a  few  cells,  not  strikingly 
different  from  the  others,  exceed  their  fellows  in  irritability 
and  contractility  and  seem  to  serve  as  guides  or  indicators, 
by  which  the  movements,  especially  of  the  tentacles,  are 
directed.  The  division  of  labor  is  not  complete  and  the 
action  is  scarcely  if  anything  more  than  a  manifestation 
of  tropism. 

Reflex  Action. — The  specialization  of  irritability  and 
conductivity  in  nervous  tissue  would  be  to  no  purpose, 
however,  if  no  means  were  provided  for  utilizing  these 
special  qualities.  A  nerve  cell  by  itself  can  do  nothing. 
To  be  of  use  in  the  performance  of  the  functions  pre- 
scribed— i.  e.,  direction,  regulation,  communication,  and 
coordination — it  must  somehow  get  into  relation  with  the 
parts  to  be  directed,  regulated,  intercommunicated,  and 
coordinated,  and  to  do  this  certain  morphological  de- 
velopment is  necessary.  This  eventuates  in  a  cell  of 
peculiar  form  known  as  a  neuron,  which  constitutes  the 
essential  structural  element  of  the  nervous  system.  It 
consists  of  a  body  and  certain  processes,  one,  supposedly 
of  greatest  importance,  called  the  neuraxis,  the  others, 
dendrites. 

The  neuraxis — a  few  cells  possess  two — forms  a  rela- 
tively long  filament  whose  function  is  to  collect  impulses 
and  bring  them  to  the  cell,  or  to  convey  impulses  originat- 
ing in  the  cell  to  the  structure  upon  which  it  acts.  The 
dendrites,  of  which  there  are  a  number,  are  relatively 


THE    HIGHER    ORGANISMS 


151 


Medullary  sheath 


—    Dendrite 


Collateral  branch 
Neurilemma 

Node  of  Ranvier 


Axis  cylinder  of 

medullated 
nerve-fiber 


Motor  ending  JK. 


Muscle-fibers 


FlG  5.5  —Diagram  of  peripheral  motor  neurone,  showing  the  specialized 
contractile  tissue,  the  muscle,  the  specialized  conducting  tissue,  the  nerve  fibre, 
with  the  motor  endings  in  the  muscle,  and  the  source  of  the  stimulation,  the 
nerve  cell.  (Bohm,  Davidoff  and  Huber.) 


152  BIOLOGY:  GENERAL  AND  MEDICAL 

short  and  finely  branched,  and  serve  to  effect  communica- 
tion between  the  nerve  cells  themselves. 

The  afferent  neuraxes  begin  in  specialized  structures, 
differing  according  to  the  particular  stimuli  they  are  de- 
signed to  collect,  and  diverging  widely  in  form  in  the  dif- 
ferent sense  organs.  The  efferent  neuraxes  terminate  in 
structures  that  differ  as  widely  according  to  the  particular 
kind  of  tissue  they  are  to  excite.  The  neuraxes  constitute 
the  bonds  of  communication  between  the  cells,  the  sources 
of  stimulation,  and  the  parts  affected. 

The  reflex  circuit  is  foreshadowed  in  what  is  seen  in  some 
of  the  higher  ccelenterates,  where  certain  ectodermal  cells 
appear  to  transmit  such  impulses  as  they  receive  to  spe- 
cialized nerve  cells,  which,  in  turn,  excite  muscle  cells  to 
action  through  the  intermediation  of  certain  fibres  ex- 
tending from  one  to  the  other.  Here  one  nerve  cell  re- 
ceives the  impulse  and  passes  it  on,  but  in  higher  animals 
this  rarely'  if  ever  occurs,  because  the  neuraxis  of  the  nerve 
cell  receiving  the  impulse  ends  in  that  cell  which  must 
pass  it  on  through  its  dendrites  to  one  or  more  neighboring 
nerve  cells  whose  neuraxes  extend  to  the  parts  to  be  af- 
fected. 

Three  elements  are  then  concerned  in  reflex  action,  and 
constitute  the  reflex  arc  or  reflex  circuit.  They  are  known 
as  the  receptor,  the  controller,  and  the  effector. 

The  receptor  is  the  specialized  beginning  of  an  afferent 
neuraxis,  and,  as  has  been  said,  is  diversified  in  form  and 
position  according  to  the  source  and  nature  of  the  im- 
pulses it  has  to  collect.  Sherrington  recognizes  three 
types  of  receptors: 

Interoceptors  or  visceral  receptive  organs,  which  only  respond  to 
stimulations  arising  within  the  body,  chiefly  in  connection  with 
the  process  of  nutrition,  excretion,  etc. 

Exteroceptors  or  somatic  sense  organs,  which  respond  to  stimula- 
tions arising  from  objects  outside  of  the  body. 

Proprioceptors  or  sense  organs  found  in  the  muscles,  tendons, 
joints,  etc.,  to  regulate  the  movements  called  forth  by  the 
stimulation  of  the  exteroceptors. 

"The  proprioceptive  sense  organs  are  deeply  embedded  in 
the  tissues  and  are  typically  excited  by  those  activities  of  the 


THE   HIGHER   ORGANISMS  153 

body  itself  which  arise  in  response  to  external  stimulation. 
The  proprioceptors  then  excite  to  reaction  the  same  organs  of 
response  as  the  exteroceptors  and  regulate  their  activity  by 
reinforcement  or  by  compensation  or  by  the  maintenance  of 
muscular  tone.  All  actions  concerned  with  motor  coordination 
and  with  the  maintenance  of  posture  or  attitude  of  the  body 
and  with  equilibrium  involve  the  proprioceptive  system." 

The  controller  is  a  nerve  center.  It  consists  of  a  group 
of  nerve  cells  communicating  with  one  another  by  den- 
drites. 

The  effector  is  the  nerve  ending  in  muscle,  gland,  or 
other  tissue  to  be  activated. 

In  every  reflex  arc  or  circuit  the  parts  must  be  so  con- 
nected that  a  stimulation  is  followed  by  a  useful  and 
adaptive  response.  The  response  is  called  a  reflex  or 
reflex  act,  and  can  be  exemplified  by  the  sudden  jerking 
away  of  the  hand  when  pricked  by  a  needle. 

Such  an  effect  is  only  possible  when  a  sensory  nerve 
ending  in  the  hand  (receptor)  is  irritated  by  the  needle, 
the  impulse  transmitted  to  a  nerve  cell  along  the  neuraxis, 
then  transmitted  to  other  nerve  cells  by  the  dendrites, 
and  then  new  impulses  sent  out  along  efferent  neuraxes 
from  these  cells  to  the  muscles  moving  the  hand  and  arm. 

How,  one  may  ask,  does  such  a  withdrawal  differ  from 
the  withdrawal  of  its  pseudopodia  by  an  irritated  ameba? 
Only  in  the  more  complicated  and  more  systematic 
manner  in  which  it  is  accomplished.  Verworn  sees  no 
essential  difference.  He  says: 

"  The  most  primitive  form  of  a  reflex  arc  exists  in  unicellular  or- 
ganisms, the  cell  body  of  which  possesses  both  the  sensory  and  motor 
elements  and  even  functions  also  as  the  control  bond  for  the  two. 
.  .  .  What  is  in  the  unicellular  organism  accomplished  within  a 
single  cell  is,  in  animals  with  a  nervous  system,  distributed  among 
several  cells.  In  the  most  simple  nervous  systems,  such  as  are  seen 
among  the  invertebrates,  one  cell,  the  sensory,  receives  the  stimulus; 
from  this  a  centripetal  nerve  path  conducts  to  a  central  cell,  the 
ganglion  cell,  and  from  here  a  centrifugal  nerve  path  conducts  to  a 
cell  that  performs  the  reaction,  the  motor  end  cell." 

The  only  practical  difference,  therefore,  between  a 
tropin  and  a  reflex  reaction  is  to  be  found  in  the  greater 


154 


BIOLOGY:  GENERAL  AND  MEDICAL 


definiteness  and   complexity   of   the   latter — really  only 
matters  of  structural  complexity  and  differentiation. 

As  the  phylogenetic  scale  is  ascended  the  complexity  of 
the  reflex  circuit  becomes  greater  and  greater  by  the  con- 
tinuous addition  of  more  and  more  nerve  cells  with  which 


Chain  reflex. 


Complex  system  illustrating  allied  and 
antagonistic  reflexes  and  physiolog- 
ical resolution. 


Complex  system  illustrating  allied  and 
antagonistic  reflexes  with  a  final 
common  path. 


Complex  system  illustrating  the  mechanism  of  a  physiological  association. 

FIG.  56. — Diagrams  representing  the  relations  of  neurons  in  five  types  of  reflex 
arcs:  A,  A.  Association  neurons;  C,  C",  C",  Cl,  and  C2,  centers  (adjusters);  E,  E', 
E",  El,  and  ES,  effectors;  FCP,  final  common  path;  R,  R',  R",  Rl,  and  R2,  recep- 
tors. (Herrick,  Introduction  to  Neurology.) 

the  controllers  communicate  through  their  dendrites,  un- 
til in  mammals  a  simple  reflex  act  is  difficult  to  detect. 
A  simple  excitation  carried  to  one  nerve  cell  is  transmitted 
to  so  many  that  a  very  wide-spread  effect  may  result. 

Herrick  recognizes  five  types  of  reflex  circuits  which  can 
be  understood  by  reference  to  his  diagrams. 


THE    HIGHER    ORGANISMS 


155 


The  specialized  nerve  cells  become  more  definitely 
localized  as  we  ascend  among  the  lower  types  of  animals, 
and  early  concentrates  in  a  more  or  less  centralized 
situation. 

In  the  unsegmented  worms  such  an  aggregation  of 
nerve  cells  at  the  anterior  end  seems  to  mark  the  begin- 
ning of  the  appearance  of  a  "brain."  From  it  two  lateral 
nerve  trunks  extend  to  the  tail,  becoming  smaller  as  they 
recede  from  the  "brain,"  apparently  through  the  loss  of 
fibers  that  are  given  off  to  the  muscles  in  due  course. 

In  the  segmented  worms,  in  addition  to  the  "brain," 
there  is  a  pair  of  nervous  ganglia  for  each  segment,  con- 
necting with  the  "brain,"  with  one  another,  and  with  those 
of  adjoining  segments  by  delicate  bundles  of  fibres.  In 


Fio.  57. — Diagram  to  express  the  fundamental  structure  of  an  arthropod,  a, 
antenna;  al,  alimentary  canal;  b,  brain;  d,  dorsal  vessel;  ex,  exoskeleton;  I, 
limb;  n,  nerve  chain;  s,  subo3sophageal  ganglion.  (After  Schmeil.) 


such  creatures  each  segment  may  be  said  to  possess  its 
own  "brains,"  though  the  anterior  pair  of  ganglia  often 
pass  into  one  mass  and  constitute  the  "chief  brain."  The 
latter  is,  however,  by  no  means  indispensable  and  can  be 
regenerated  should  it  be  lost,  which  it  probably  frequently 
is  in  earthworms,  whose  heads  are  pulled  off  by  birds. 

This  general  arrangement  of  a  double  chain  of  com- 
municating and  intercommunicating  ganglia,  running  from 
end  to  end  of  the  body,  and  increasing  in  complexity  and 
importance  anteriorly  when  the  greatest  number  of 
special  senses  connect  with  it,  and  elsewhere  in  proportion 
to  the  position  and  importance  of  the  limbs  or  appendages, 
constitutes  the  foundation  of  the  central  nervous  system 
which  undergoes  progressive  development  with  increasing 


156  BIOLOGY:  GENERAL  AND  MEDICAL 

complexity  until  it  culminates  in  the  great  brain  and 
spinal  cord  of  man,  enclosed  in  the  skull  and  spinal 
column  for  protection. 

As  the  greatest  development  of  the  nervous  system  takes 
place  primarily  where  the  nerves  of  the  special  senses  are 
received,  we  must  look  upon  their  appearance  and  develop- 
ment as  contributory  to  the  development  of  the  central 
nervous  system,  and  before  progressing  further  with  its 
functions  pause  to  consider  how  and  why. 

Metazoan  organisms  are  rarely  so  simple  in  structure 
as  not  to  be  provided  with  some  kind  of  outer  protection — 
cuticle,  shell,  skin,  etc. — the  purpose  of  which  is  to  pre- 
vent the  interruption  of  the  normal  activity  of  the  internal 
cells  with  beginning  specialization  of  function,  by  contact 
with  external  forces.  If  cells  are  to  perform  one  function 
to  the  exclusion  of  others,  they  must  be  saved  from  the 
necessity  of  performing  those  others,  and  to  this  end  the 
cuticle,  composed  of  elements  freed  from  the  customary 
irritability  of  protoplasm,  was  developed.  This,  however, 
was  not  without  its  disadvantages,  for  not  only  is  protec- 
tion thereby  afforded  against  influences  that  are  injurious, 
but  quite  as  much  against  those  that  are  beneficial.  Spe- 
cial adaptations  for  appreciating  the  useful  influences 
therefore  made  their  appearance  in  the  organs  of  special 
sense,  each  of  which  consists  essentially  of  specialized 
protoplasm  which  is  highly  sensitive  to  some  particular 
form  of  energy  manifestation,  but  relatively  insensitive  to 
other  forms  of  stimulation.  Each  sense  organ  possesses, 
in  addition,  certain  accessory  parts  adapted  to  concen- 
trate the  stimuli  upon  the  essential  sensitive  protoplasm, 
to  intensify  the  form  of  the  stimulus,  or  to  so  transform  the 
energy  of  the  stimulus  as  to  enable  it  to  act  more  efficiently 
upon  the  essential  end-organ. 

Herrick  says: 

"We  may  conceive  the  body  as  immersed  in  a  world  full  of  energy 
manifestations  of  diverse  sorts,  but  more  or  less  completely  insulated 
from  the  play  of  these  cosmic  forces  by  an  impervious  cuticle.  The 
body  surface  is,  however,  permeable  in  some  places  to  these  en- 
vironing forces,  and  in  a  definite  fashion,  one  part  responding  to  a 


THE   HIGHER   ORGANISMS  157 

particular  series  of  vibrations,  another  part  to  a  different  series. 
.  .  .  The  sense-organs  may  be  compared  with  windows  each  of 
which  opens  out  into  a  particular  field  so  as  to  admit  its  own  series 
of  environmental  forces.  In  each  species  of  animal  these  windows 
are  arranged  in  a  characteristic  way,  so  as  to  admit  only  those  forms 
of  energy  which  are  of  practical  significance  to  the  animal  as  it  lives 
in  its  own  natural  environment." 

These  "windows"  of  Herrick's  comprise  the  organs  of 
special  sense— touch,  taste,  scent,  sight,  hearing,  etc., 
and  are  recognized  as  feelers,  tongues,  noses,  eyes,  ears, 
etc.  They  all  admit  vibrations  to  the  central  nervous 
system.  Without  them  higher  animals  would  be  unable 
to  exist  but  for  a  short  time,  during  which  they  would 
seem  to  themselves  to  float  in  space  in  eternal  silence  and 
darkness. 


-    Nerve-fibre  "~ —  "     Nerve-fibre 


Capsule 


-     Nerve-fibre 
_   Nerve-fibre 


Fio.  58. — Meisner's  corpuscle  from  man;  X  750.     (Bohm,  Davidoff,  and  Huber.) 

Touch. — The  tactile  sense  can  be  traced  to  the  irritabil- 
ity of  living  substance.  It  begins  without  special  organs 
as  the  phenomenon  of  thigmotropism.  The  pseudopods 
of  the  rhizopoda  are  thigmotropic,  hence  tactile  and 
discriminating.  But  in  composite  organization  it  is  not 
sufficient  that  the  cells  shall  be  equally  irritable  and 
similarly  impressed  by  external  agents.  Division  of 


158 


BIOLOGY:  GENERAL  AND  MEDICAL 


labor  begins,  and  it  becomes  necessary  for  a  more  elabo- 
rate response  to  follow  certain  stimuli  than  could  be 
effected  by  cells  acting  individually.  Moreover,  certain 
cells  are  so  situated  as  to  be,  above  their  fellows,  suscep- 
tible to  external  agents,  so  that  we  need  only  ascend  to 
the  ccelenterates  to  find  the  ectodermal  cells  more  sensi- 
tive than  others,  and  to  find  a  mechanism  by  which  the 
external  impressions  are  communicated  to  groups  of 
cells,  by  which  they  are  to  be  utilized,  through  inter- 
mediate nerve  cells.  Though  the  sensory  apparatus  is 


FIG.  59.  — Diagrams  showing  some  of  the  stages  in  the  increasing  complexity 
of  the  simple  eye  in  invertebrates.  A,  Simple  pigment  spot  in  epithelium  have 
nerve-endings  associated  with  pigment  cells  (as  in  some  medusae) ;  B,  pigment 
cells  in  a  pit-like  depression  (as  in  Patella) ;  C,  with  pin-hole  opening  and  vitreous 
humor  in  cavity  (as  in  Trochus) ;  D,  completely  closed  pit,  with  lens  and  cornea 
(as  in  Triton  and  many  other  Mollusks);  E,  pigment  area  elevated  instead  of 
depressed  lens  of  thickened  cuticula  (as  in  the  medusa,  Lizzia);  F,  retinal  cells 
more  highly  magnified,  ep,  Epidermh;  /,  nerve  fibre;  I,  lens;  op,  optic  nerve; 
p,  pigment  cells;  r,  retina;  v.h,  vitreous  humor.  (Galloway.) 


so  simple  that  it  is  difficult  to  account  for  all  that  is 
accomplished,  it  already  has  discriminative  powers. 
Useful  objects  when  touched  are  apprehended  by  the 


THE   HIGHER   ORGANISMS  159 

tentacles  of  the  coelenterates,  indifferent  objects  are  ne- 
glected, harmful  objects  may  be  avoided. 

So  soon  as  a  central  nervous  system  appears,  nerve 
fibres  connect  the  sensitive  cells  upon  the  surface  with 
controlling  centres,  and  more  exact  conduction  and 
distribution  of  impressions  become  possible.  The  per- 
ipheral apparatus  continues  to  specialize  until  the  sense 
of  contact  is  capable  of  differentiation  from  the  sense 
of  harmful  contact  or  pain,  and  eventually  into  a  great 
variety  of  impressions,  pleasurable,  painful,  thermic, 
etc.  But  such  development  is  not  possible  until  there 
appear  in  the  superficial  tissues  highly  specialized  nerve 
endings  (Meissner's  corpuscles)  adapted  to  the  recep- 
tion of  the  particular  impressions,  and  in  the  central 
nervous  system  that  complex  adjustment  of  white  and 
gray  matter  by  which  the  impressions  are  received  and 
appreciated. 

Sight. — If  we  define  sight  as  the  ability  to  appreciate 
light,  we  must  admit  that  it  makes  its  appearance  in  the 
most  simple  form  as  the  phototropic  sensitivity  of 
protoplasm.  If,  however,  we  mean  by  the  term,  the 
recognition  of  light  by  the  aid  of  an  eye,  we  are  almost 
as  badly  off  because  of  the  uncertainty  as  to  what  shall 
constitute  an  eye.  In  its  broadest  sense,  an  eye  is  an 
organ  formed  for  the  appreciation  of  light,  to  the  waves 
of  which  it  is  specially  sensitive.  Such  an  organ  may 
be  extremely  simple  in  structure,  totally  devoid  of  the 
power  of  forming  images  of  external  objects,  and  con- 
sist merely  of  a  group  of  pigmented  cells,  as  for  example, 
the  eye-spot  of  Euglena  or  the  eyespots  in  the  higher 
coelenterates.  The  latter  connect  with  subjacent  nerve 
cells,  thus  forming  a  true  sensory  organ,  though  in  what 
manner  the  impressions  received  are  utilized  is  difficult 
to  understand. 

Similar  eyes  are  also  found  in  the  flat  worms,  though  in 
these  the  nervous  elements  become  more  numerous,  and 
instead  of  being  upon  the  surface  of  the  body,  the  organ 
becomes  situated  in  a  more  or  less  pronounced  pit  or 
depression  from  which  the  nervous  cells  radiate,  the 


160  BIOLOGY:  GENERAL  AND  MEDICAL 

fibres  with  which  they  are  connected  converging  to 
form  an  optic  nerve.  Eyes  of  this  kind  persist  among 
the  arthropods,  though  more  highly  specialized  eyes 
are  also  present. 

From  such  eyes  it  is  a  simple  step  to  the  camera-eye,  in 
which  the  nervous  elements  surround  a  spherical  space 
into  which  the  light  comes  through  a  minute  opening, 
the  homologue  of  the  pupil,  causing  an  inverted  image 
to  fall  upon  the  more  numerous  nerve  elements  and 


Via.  60. 


FIG.  60. : — Diagram  illustrating  the  early  development  of  the  vertebrate  eye, 
(Galloway.) 

b.v.  The  brain  vesicle  formed  by  the  invagination  of  the  ectoderm,  eel.;  mes, 
mesodermal  tissue;  os,  optic  stalk;  ov,  optic  vesicle,  a  portion  of  the  brain 
vesicle;  I,  lens.  The  right  side  of  the  figure  shows  a  slightly  later  developmental 
stage  than  the  left. 

FIG.  61. — Diagram  illustrating  a  later  developmental  stage  of  the  vertebrate 
eye.  On,  optic  nerve;  r,  retina;  v.h,  vitreous  humor;  I,  lens;  ect,  ectodermal  tissue; 
mes,  mesodermal  tissue.  (Galloway.) 

so  effecting  a  differentiation  of  lights  and  shadows. 
The  next  specialization  consists  in  an  outer  transparent 
cuticular  covering  or  cornea,  the  presence  of  a  clear  jelly 
in  the  space — vitreous  body — and  eventually  of  a  lens 
by  which  the  light  rays  are  refracted  and  accurately 
distributed.  There  are  many  variations  of  the  ap- 
paratus, however,  for  in  the  arthropods  it  develops  into 
a  congeries  of  what  might  be  described  as  visual  units, 
as  in  the  compound  eye  of  the  insects  which  are  made 
up  of  hundreds  of  units  consisting  of  an  outer  ectodermal 
transparent  cuticle  or  cornea,  beneath  which  are  pig- 


THE    HIGHER    ORGANISMS  161 

ment  cells  with  subsequent  nervous  elements  in  groups. 
The  images  gathered  by  such  eyes  may  be  regarded  as  a 
kind  of  mosaic  made  up  of  many  small  bits.  There 
can  be  no  accommodation  and  no  perspective.  From 
such  eyes  great  bundles  of  nerve  fibres  pass  to  the 
optic  lobes  of  the  brain,  so  increasing  its  complexity. 
Among  certain  mollusks,  such  as  the  cephalopods,  the 
eye  forms  a  beautiful  and  striking  organ  superficially 


aperture 


FIG.  62. — Section    through    the   otocyst   of   arenicola.     (After   Ashworth   and 
Gamble.) 

resembling  the  vertebrate  eye,  but  having  certain  essen- 
tial differences,  for  the  retinal  nerve  cells  are  directed 
toward  the  centre  of  the  globe  and  are  outside  of  the 
pigment  layer,  while  in  the  more  perfect  vertebrate 
organ  the  nerve  endings  are  directed  away  from  the 
centre  and  the  pigment  layer  of  the  retina  is  outside. 

As  the  structure  of  the  eye  increases  in  perfection  the 
number  of  nervous  elements  increases  greatly,  and  the 
complexity  of  the  central  nervous  system  is  increased 
both  by  the  increased  number  of  fibres  it  receives  and 
the  number  of  cells  with  which  they  communicate,  so 
that  the  new  centres,  optic  lobes,  and  optic  thalami  make 
their  appearance. 

Hearing. — In  this  sense  we  doubtless  have  to  do  with 
11 


162  BIOLOGY:  GENERAL  AND  MEDICAL 

a  specialization  of  thigmotropic  irritability  to  vibrations 
set  up  in  the  media  in  which  the  organism  lives.  The 
inception  of  the  organs  by  which  such  vibrations  are 
originally  recognized  is  unknown.  The  first  organs 
that  can  be  definitely  made  out  are  found  among  the 
coelenterates.  In  certain  jelly-fishes  minute  vesicles 
are  found  situated  at  the  edge  of  the  disc,  each  contain- 
ing one  or  "more  small  crystals  or  concretions.  These 
vesicles  are  known  as  otocysts,  the  crystals  as  otoliths. 
Similar  organs,  by  which  vibrations  are  caught  and  trans- 
mitted to  the  central  nervous  system,  occur  among 
worms  and  mollusks.  They  are  usually  minute,  difficult 
to  find,  and  may  be  situated  in  unexpected  places,  as, 
for  example,  in  the  clam,  where  they  occur  in  the  so- 
called  "foot."  In  the  Crustacea  they  are  extremely 
peculiar  and  consist  of  small  sacs  formed  by  an  invagi- 
nation  of  the  integument,  filled  with  fluid,  provided  with 


FIG.  63. —Transverse  vertical  section  of  Corti's  organ  of  a  man  twenty-nine 
years  old.  es,  Limbus  laminse  spiralis;  me,  membrana  tectoria;  Hb,  H onsen's 
strise;  mf,  fibres  of  attachment  of  the  membrana  tectoria  to  the  zona  tecta;  si, 
sulcus  spiralis;  siz,  epithelium  of  the  sulcus  spiralis;  is,  inner  supporting  cells; 
ic,  inner  rod  cells  in  connection  with  the  outer  rod  cells,  between  which  is  seen 
the  tunnel  «)  of  Corti;  ih,  inner  hair  cell;  ah^-dh4,  outer  hair  cells;  dz,  Deiters' 
cells;  as,  Hensen's  supporting  cells;  rb,  nerve  fibres  of  the  ramulus  basilaris; 
n1— 7i6,  outer  bundles  of  the  spiral  nerve  fibres;  rf,  radiating  tunnel  fibres;  at 
inner  part  of  Nuel's  space;  mb,  upper  layer  of  the  membrana  basilaris;  mb', 
lower  layer  of  the  membrana  basilaris;  tb,  layer  covering  the  tympanic  surface 
of  the  membrana  basilaris;  Us,  ligamentum  spirale.  (Gruber,  after  Retzius.) 


many  small  hair-like  projections.  Grains  of  sand  entering 
from  the  exterior  seem  to  be  essential  to  the  perfection 
of  the  apparatus,  though  sometimes  concretions  of  lime 
salts  form  in  the  organ  and  constitute  an  improvement. 


THE    HIGHER    ORGANISMS 


163 


An  advance  in  the  development  of  the  apparatus  is 
found  among  the  insects  whose  "ears"  or  chordo- 
tonal  organs  are  provided  with  a  tympanic  membrane. 
These  organs  are  large,  and  may  be  situated  upon  the 
sides  of  the  abdomen  or  upon  the  anterior  tibiae.  Some- 
times, however,  no  auditory  organs  can  be  found  when 
it  is  supposed  that  the  antennae  act  as  vibration-receiving 
organs  as  well  as  organs  of  touch  and  smell. 

The  vertebrate  ears  are  two  in  number,  vary  in 
elaborateness,  and  are  situated  in  special  cavities  of  the 
cranial  bones.  The  elaborate  auditory  mechanism  found 
in  mammals  is  divisible  into  an  external  ear  to  receive  the 
sound  waves,  a  middle  ear  or  "drum"  to  intensify  them, 
and  an  internal  ear  containing  the  actual  auditory  nerve 
fibres  spread  out  in  what  is  called 
the  "organ  of  Corti." 

From  the  simple  and  complex 
ears  nerve  fibres  pass  to  the  brain, 
communicating  with  special  audi- 
tory centres  and  so  increasing  its 
complexity  both  by  the  addition 
of  fibres  and  cells. 

Smell. — This  sense  may  be  re- 
garded as  an  amplification  of  chem- 
otropic  irritability,  by  virtue  of 
which  certain  superficially  situated 
cells  specialized  for  the  purpose  re- 
ceive and  transmit  chemical  im- 
pulses to  the  central  nervous 
system. 

The  primitive  means  by  which 
such  impulses  are  collected  are  en- 
tirely conjectural.  In  the  absence 
of  distinct  organs  to  which  such  a 
function  can  be  ^signed  we  are  in  gJ3,t*ilt£i 

doubt      as     tO     Whether     the    lowly      r,  rod.     (After  Hauser.) 

organisms    possess    the    sense    of 

smell    or  not.     Even    among    the    insects    no    definite 

organs   for  the   purpose    can   be    found,    and   yet  the 


n 

FIG  64  — Longitudinal 
section  of  antennal  olfac- 
tory organ  of  wasp,  Vespa; 
c,  Olfactory  cell;  en,  olfac- 


164 


BIOLOGY:  GENERAL  AND  MEDICAL 


rapidity  with  which  bees  find  honey  and  certain  carrion 
insects  their  concealed  food  suggest  that  such  insects 
possess  this  sense  to  a  high  degree.  The  antennae  seem 
to  be  the  olfactory  organs  and  possess  nerve  endings 
supposed  to  be  organs  of  scent,  though  certain  organs 
at  the  base  of  the  wings  in  some  flies  and  upon  the  caudal 
appendages  of  other  flies  may  be  olfactory  in  nature. 

Among  vertebrates  the  sense  of  smell  is  always  situated 
in  the  nose,  upon  the  mucous  membranes  of  which  the 
olfactory  nerves  distribute  in  varying  number  according 
to  the  activity  of  the  sense.  These  nerves  communicate 
with  the  olfactory  lobes  of  the  brain  and  constitute  an 
added  source  of  complexity  to  that  organ. 

Taste. — This  is  another  amplification  and  specializa- 


Process  of 
neuro-epi 

Epithe-    Nerve-          thelial     Taste- 
Hum,      fibrils.          cell.  pore. 


Sustentacular 
cell. 


Terminal   . 
branches  of 
nerves. 

FIG  65. — Schematic   representation  of  a  taste-goblet.   (Bohm,   Davidoff,  at 
Huber.) 


tion  of  the  chemotropic  irritability  of  protoplasm.  As 
in  the  highest  animals,  this  sense  resides  in  certain  dis- 
tributed nerve  endings  in  the  tongue  and  palate,  does 
not  take  the  form  of  a  visible  sensory  organ,  and  so  is 


THE    HIGHER    ORGANISMS  165 

not  easy  to  localize.  Little  can  be  learned  about  it  in 
animals  whose  structure  is  essentially  dissimilar. 

It  would  seem  as  though  it  must  be  almost  universal 
among  animals  as  the  chief  means  of  discriminating 
between  what  is  useful  and  what  is  useless  as  food,  yet 
in  this  regard  it  is  easy  to  fall  into  error  for  in  man  we 
find  the  taste  an  unsafe  guide,  many  things  not  pleasant 
to  the  palate  being  serviceable  for  food  and  some  that 
taste  quite  agreeably  being  injurious  or  even  poisonous. 
It  may  be,  therefore,  that  taste  is  not  a  common  sense, 
and  that  other  means  of  discriminating  between  useful 
and  useless  things  are  provided.  However  this  may 
be,  when  the  sense  exists  there  must  be  specialized  nerve 
endings  to  be  impressed,  fibres  to  convey  these  impres- 
sions to  the  brain,  and  centres  where  they  are  to  be 
received  and  retained  or  utilized,  and  the  fact  holds 
good  that  with  each  sense  the  general  complexity  of  the 
central  nervous  system  is  increased. 

The  Central  Nervous  System. — The  greater  number  of 
the  special  senses  are  situated  in  the  cephalic  region.  As 
in  the  usual  modes  of  progress,  whether  creeping,  walking, 
flying,  or  swimming,  the  head  or  cephalic  end  of  the  animal 
precedes  the  rest ;  it  is  the  first  to  come  under  the  influence 
of  external  forces,  and  therefore  the  most  favorable  situa- 
tion for  those  "windows"  through  which  impressions  are 
to  be  received.  The  result  is  quite  obvious:  with  the 
perfection  of  the  organs  of  special  sense  and  their  correla- 
tion with  one  another,  the  adjustment  of  reflexes  and  the 
formation  of  associations,  memory  and  the  higher  ner- 
vous functions,  the  ganglionic  masses  at  the  cephalic 
region  come  to  preponderate  over  all  others  in  size  and 
importance,  and  form  the  brain. 

As  has  been  shown,  the  brain  is  important  only  as  it 
becomes  more  and  more  highly  specialized  and  the  seat 
of  the  governing  forces  that  control  the  vital  functions. 
The  loss  or  destruction  of  the  extremely  simple  brain  of 
a  worm  or  even  of  a  mollusk  may  be  followed  by  regenera- 
tion in  part  or  altogether;  the  destruction  or  removal  of 


166  BIOLOGY:  GENERAL  AND  MEDICAL 

the  brain  of  a  batrachian,  though  it  cannot  be  followed 
by  any  regeneration,  does  not  prevent  the  animal  from 
living  until  it  starves  to  death,  but  the  destruction  of  the 
brain  of  a  mammal  is  commonly  followed  by  almost  in- 
stant death.  As  it  becomes  one  of  the  most  complex  of 
organs,  it  becomes  one  of  the  most  indispensable. 

The  complexity  of  structure  depends  upon  a  vast  in- 
crease in  the  number  of  nerve  cells,  the  separation  of 
many  of  these  into  groups  possessing  specific  functions, 
and  the  formation  of  connecting  paths  or  tracts,  through 
which  the  communicating  neuraxes  pass  in  orderly  ar- 
rangement from  cell-group  to  cell-group.  The  resulting 
localization  of  cerebral  function  in  these  cell-groups  and 
the  intercommunication  of  the  many  cells  in  the  groups 
with  one  another  and  with  the  cells  in  other  groups 
through  neuraxes  and  dendrites  affords  an  explanation  of 
the  greater  plasticity  of  behavior  in  higher  animals  as  con- 
trasted with  the  lower. 

Thus,  if  a  worm,  with  a  very  simple  nervous  system 
composed  of  little  more  than  simple  reflex  circuits,  be 
touched,  the  sensory  receptors  in  its  cuticle  transmit  to 
the  controlling  center  an  impulse  that  is  passed  on  to 
several  muscle  fibres,  which  contract,  causing  the  worm 
to  wriggle.  In  all  probability  no  greater  response  is  pos- 
sible because  the  simplicity  of  the  nervous  communication 
makes  no  provision  for  it. 

If  the  general  structural  complexity  of  an  animal  en- 
ables it  to  perform  more  complicated  movements,  the 
nervous  system  usually  shows  a  corresponding  complexity, 
with  the  result  that  the  reflexes  involve  a  more  complex 
and  coordinated  response.  This  is  well  exemplified  by 
the  following  experiment  upon  a  frog.  The  head  of  the 
animal  is  cut  off  or  its  brain  entirely  destroyed.  A  small 
piece  of  blotting-paper  moistened  with  acetic  acid  is 
placed  upon  the  inner  surface  of  one  hind  leg  which  is 
held  to  prevent  the  simple  reflex  of  withdrawal.  In 
many  cases,  after  a  brief  interval,  the  animal  brushes  the 
paper  away  with  the  opposite  foot.  Here  is  a  very  com- 


THE   HIGHER   ORGANISMS  167 

plicated  reflex  involving  the  correlated  action  of  many 
muscles,  and  effected  solely  by  stimuli  conveyed  to  spinal 
ganglia  and  impulses  sent  out  by  them.  There  can  be  no 
doubt  about  the  purpose  of  the  reflex:  it  was  to  protect 
the  skin  from  the  injurious  effect  of  the  acid.  When  we 
examine  the  character  of  reflexes  in  general,  we  find  them 
purposeful,  as  a  rule,  yet  destined  to  a  purpose  of  which 
the  animal  can  be  no  more  conscious  than  it  is  of  the 
performance  of  the  act,  which,  of  course,  it  cannot  be  when 
its  head  is  cut  off  or  its  brain  destroyed. 

Thus  unconsciousness  of  the  act,  at  the  time  of  its 
commission,  is  a  characteristic  of  reflex  action. 

Various  definitions  of  reflex  action  are  given,  but  usu- 
ally agree  in  stating  that  it  is  the  unconscious  and  invol- 
untary response  that  follows  the  stimulation  of  afferent  nerve 
fibres.  Sherrington  and  Parmelee  define  reflexes  as  "ac- 
tions in  which  there  follows  on  an  initiating  reaction,  an 
end-effect  reached  through  the  mediation  of  a  conductor 
itself  incapable  either  of  the  end-effect  or,  under  normal 
conditions,  the  inception  of  the  reaction."  According  to 
Jennings  it  must  be  an  invariable  response,  but  the  in- 
variability of  the  action  should  not  be  too  much  empha- 
sized, as  among  higher  animals  the  numerous  bonds  of 
communcation  between  the  nerve  centres  permit  of  con- 
siderable variation. 

If  two  frogs,  one  with  its  telencephalon  or  highest  brain 
center  destroyed,  the  other  normal,  be  dropped  into  hot 
water,  each  will  immediately  climb  out  if  there  be  any- 
thing to  climb  upon;  but  if  the  two  frogs  are  placed  in  a 
vessel  of  water  which  is  gradually  heated,  the  normal  frog 
will  climb  out  when  the  water  reaches  a  certain  tempera- 
ture, while  the  frog  without  a  brain  remains  in  the  water 
and  cooks.  To  explain  the  behavior  of  the  brainless  frog 
in  this  experiment  we  suppose  that  the  sudden  and  violent 
stimulation  caused  by  the  application  of  hot  water  to  the 
entire  skin  resulted  in  reflex  action  involving  all  of  its 
members  in  complicated  and  coordinated  manner.  When 
the  water  into  which  the  frog  is  placed  is  gradually  heated, 
however,  no  sudden  stimulation  is  experienced. 


168  BIOLOGY:  GENERAL  AND  MEDICAL 

The  suddenness  and  violence  of  the  irritation  has  much 
to  do  with  the  reflexes,  for  connected  with  many  of  the 
reflex  -  circuits  of  the  higher  animals  are  inhibitors  that 
prevent  their  occurrence  or  greatly  modify  them.  How 
awkward  it  would  be  to  find  that  the  hand  was  jerked 
away  from  every  object  it  touched!  Yet  such  might  be 
the  case  were  there  no  inhibiting  influences  governing  the 
reflex  circuits. 

If  the  brain  of  a  pigeon  be  destroyed,  the  bird  sits 
quietly  with  ruffled  feathers,  and  never  moves  unless  dis- 
turbed. If  it  be  taken  into  the  hand  and  thrown  into  the 
air,  it  immediately  spreads  its  wings  and  flies  to  some 
nearby  object  upon  which  it  perches  and  remains  quietly 
for  an  indefinite  period. 

^  How  different  are  the  brainless  frog  and  brainless  pigeon 
from  the  normal  frog  and  the  normal  pigeon.  If  the 
normal  frog  be  touched  it  jumps  into  the  water,  swims 
away,  and  hides  under  the  lily-pads;  if  the  pigeon  be 
approached  too  closely  it  flies  away.  Here  we  say  we  see 
manifestations  of  a  higher  sort,  viz.,  an  instinct  that 
prompts  the  creature  to  self-preservation,  and  as  we  did 
not  see  it  in  either  of  the  creatures  when  their  brains  were 
destroyed,  we  conclude  that  we  have  to  do  with  a  function 
whose  localization  was  in  that  part  of  the  nervous  system 
that  we  had  destroyed,  the  brain,  which  is  undoubtedly 
the  case. 

However,  we  saw  in  tropisms  the  flight  from  injurious 
agents,  we  saw  in  the  simple  reflex  the  withdrawal  from 
an  injurious  agent,  and  we  see  but  an  exaggeration  of  the 
phenomenon  in  what  are  now  to  be  discussed  as  instincts. 
When  shall  it  be  said  that  tropisms  and  end  reflexes  begin? 
When  shall  reflexes  end  and  instincts  begin? 

Instincts. — Sherrington  and  Parmelee  both  believe  in 
making  a  clear  separation  between  reflexes  and  other 
forms  of  behavior,  but  we  find  upon  examining  a  series 
of  animals,  with  the  lowest  at  one  end  and  the  highest 
at  the  other,  that  their  nervous  reactions  pass  from  the 
most  simple  to  the  most  complex,  without  thresholds  suffi- 


THE    HIGHER    ORGANISMS  169 

ciently  clear  cut  to  make  it  possible  to  say  when  one 
ends  and  another  begins.  In  such  a  series  we  observe, 
in  ascending  order,  tropisms,  reflexes,  instincts,  associa- 
tion of  ideas,  memory,  intelligence,  consciousness,  self- 
consciousness,  and  reason. 

Loeb  and  Jennings  find  it  impossible  to  clearly  separate 
tropisms,  reflexes,  and  instincts;  T.  H.  Morgan  seems  to 
regard  tropisms  and  instincts  as  identical;  Herbert 
Spencer,  who  defines  instincts  as  ' 'compound  reflex  ac- 
tion," sees  no  clear  line  of  demarkation  between  instinct 
and  reflex  action,  and  Parmelee  believes  that  there  is  a 
strict  continuity  between  all  these  different  forms  of 
behavior,  and  that  the  more  complex  are  built  up  upon 
the  simpler. 

All  of  these  manifestations  of  vital  activity  are  col- 
lectively known  as  behavior,  and  behavior  of  whatever 
kind  is  based  upon  structure.  James  says  "instincts  are 
the  correlatives  of  structure." 

As  the  structural  development  of  every  creature  is  the 
result  of  its  inheritance,  so  its  behavior  also  results  from 
heredity.  As  we  believe  that  structural  modifications 
were  gradually  developed,  so  we  believe  that  behavior 
was  gradually  developed  to  the  point  at  which  we  find  it. 
If,  however,  through  sudden  mutation  the  structure  of 
an  animal  could  suddenly  change,  it  is  conceivable  that 
its  behavior  might  change  correspondingly,  an  old  in- 
stinct being  lost  or  a  new  instinct  developing. 

This  is  born  out  by  what  we  observe  among  animals  in 
whose  life  history  distinct  changes  occur  and  require  a 
whole  new  series  of  instincts  to  determine  behavior. 
Thus  instinct  in  a  caterpillar  determines  that  it  hold 
tenaciously  to  the  twig  whose  leaves  it  eats,  move  from 
twig  to  twig  as  the  leaves  are  consumed,  eat  voraciously 
until  of  full  size,  and  then  spin  a  cocoon  of  complicated 
weaving,  and  of  a  pattern  common  to  all  its  kind,  in  which 
it  imprisons  itself  to  rest  during  the  chrysalis  stage  and 
from  which  it  emerges  an  imago.  The  imago  or  moth 
seems  to  know  nothing  of  the  instincts  of  its  larval  stage, 


170  BIOLOGY:  GENERAL  AND  MEDICAL 

but  instinctively  flies,  without  learning,  recognizes  its 
sexual  mate,  never  having  seen  one  before,  nourishes 
itself  by  sipping  the  nectar  from  flowers  with  its  long 
tongue,  and  lays  its  eggs  conveniently  to  the  vegetation 
upon  which  its  future  progeny  are  to  feed. 

All  this  is  so  perfectly  adapted  to  the  ends  of  caterpillar 
and  moth  life  as  to  make  it  appear  as  though  the  cater- 
pillar knew  that  it  was  only  by  eating  that  it  could  spin 
its  cocoon,  knew  that  it  must  spin  the  cocoon  to  protect 
itself  against  enemies  and  against  the  cold  of  winter  dur- 
ing its  pupation,  knew  that  it  would  become  a  moth, 
as  a  moth  knew  that  it  must  mate  and  lay  its  eggs,  etc., 
and  continually  impresses  us  as  being  the  result  of  con- 
scious volition  based  upon  such  knowledge.  But  the 
briefest  reflection  upon  the  matter  is  sufficient  to  show  that 
neither  the  caterpillar  nor  the  moth  can  know  the  future 
with  reference  to  any  of  the  instinctive  acts  performed. 

Romanes  held  that  instinct  always  involved  a  mental 
element,  while  reflex  action  did  not,  and  therefore  the 
mental  element  is  consciousness.  He  supposed  that  the 
time  element  was  too  short  to  permit  consciousness  in  re- 
flex action,  but  long  enough  to  permit  it  in  instinctive  ac- 
tion. To  him  instinctive  action  was  "hereditary  memory." 

It  has  been  said  that  "primary  instincts  result  from  the 
natural  selection  of  actions  which  although  never  intelli- 
gent, yet  happen  to  have  been  of  benefit  to  the  animals 
which  first  performed  them.  Secondary  instincts  result 
from  the  hereditary  transmission  of  habits  of  actions 
originally  intelligent."  George  H.  Lewes  considers  such 
instincts  to  be  cases  of  "lapsed  intelligence." 

James  defines  instinct  as  "the  faculty  of  acting  in  such 
a  way  as  to  produce  certain  ends,  without  foresight  of  the 
ends  and  without  previous  education  in  the  performance." 

Lloyd  Morgan  says,  "Instincts  are  congenital  adaptive 
and  coordinated  activities  of  relative  complexity  and  in- 
volving the  behavior  of  the  organism  as  a  whole."  Par- 
melee  defines  them  as  "inherited  combinations  of  reflexes 
which  have  been  integrated  by  the  central  nervous  system 


THE   HIGHER   ORGANISMS  171 

so  as  to  cause  external  activities  of  the  organism,  which 
usually  characterize  a  whole  species  and  are  usually 
adaptive." 

It  is  usually  characteristic  of  instinctive  acts  that  they 
are  performed  but  once,  or  if  repeated,  are  done  again  and 
again  under  the  same  conditions  without  substantial 
change.  It  even  sometimes  happens  that  any  interrup- 
tion in  the  proceeding  may  make  it  necessary  to  begin 
again  at  the  very  beginning  instead  of  continuing  from 
the  stage  of  interruption.  The  act  is  automatic  rather 
than  rational  and  consecutive.  Thus,  when  the  Peck- 
hams,  on  one  occasion,  stole  a  spider  that  a  female  Pom- 
pilus  quinquenotutus  was  about  to  carry  into  her  sub- 
terraneous nest,  the  insect,  discovering  its  loss,  repaired 
it  by  capturing  a  new  spider,  for  which  it  then  proceeded 
to  dig  another  nest  instead  of  utilizing  that  previously 
prepared  and  now  abandoned. 

Careful  observation  of  the  multitudinous  and  multi- 
farious forms  of  instinctive  action  among  animals  results 
in  the  conclusion  that  instinct  consists  of  serial  and 
elaborately  coordinated  reactions,  to  stimuli  arising  within 
or  without  the  animal,  occurring  in  regular  order  and 
eventuating  in  fairly  uniform  results. 

That  the  end-results  of  the  instinctive  acts  of  animals 
of  the  same  kind  are  not  uniform  and  invariable  is  no  more 
than  is  to  be  expected  when  one  considers  that  the  animals 
themselves,  though  of  the  same  species,  are  not  invariable. 
In  morphology  the  variations  show  themselves  in  size, 
strength,  brilliance  of  coloring,  agility,  etc.;  in  their  be- 
havior the  differences  appear  as  carefulness  and  careless- 
ness in  the  details  of  performance. 

Many  observers,  however,  find  it  difficult  to  escape  the 
conviction  that  associated  with  instinctive  behavior  there 
is  at  least  a  glimmering  of  intelligence.  The  animal 
seems  to  appreciate  the  fact  that  it  is  doing  something 
and  to  know  what  it  is  about.  This  brings  us  to  a  brief 
consideration  of  intelligence. 

Intelligence. — Intelligence  is  shown  by  the  adaptation 


172  BIOLOGY:  GENERAL  AND  MEDICAL 

of  behavior  to  new  conditions.  It  enables  the  animal  to 
successfully  overcome  difficulties  arising  in  the  perform- 
ance of  behavior,  and  results  in  new  forms  of  behavior. 

According  to  Parmelee,  "it  is  made  up  of  tropic,  reflex, 
and  instinctive  actions  that  have  been  combined  in  new 
ways  as  a  result  of  experience  so  as  to  constitute  new  forms 
of  behavior." 

If  it  is  conceded  to  depend  upon  experience,  it  must 
have  its  foundation  in  the  recollection  of  the  results  of 
previous  trials,  i.  e.,  memory,  or,  to  be  more  correct,  in 
associative  memory. 

Loeb  defines  this  as  "that  mechanism  by  which  a  stim- 
ulus brings  about  not  only  the  effects  which  its  nature 
and  the  specific  structure  of  the  irritable  organ  calls  for, 
but  by  which  it  brings  about  also  the  effects  of  other  stimuli 
which  formerly  acted  upon  the  organism  almost  or  quite 
simultaneously  with  the  stimulus  in  question." 

Such  associative  memory  is  the  basis  of  training;  any 
animal  that  can  be  trained  has  associative  memory,  and 
training  is  nothing  more  than  the  modification  of  behavior 
through  experience. 

"Processes  which  occur  in  the  central  nervous  system 
leave  an  impression  or  trace  by  which  they  can  be  repro- 
duced even  under  different  circumstances  than  those 
under  which  they  originated.  .  .  .  Two  processes  which 
occur  simultaneously  or  in  quick  succession  will  leave 
traces  which  fuse  together,  so  that  if  later  one  of  the 
processes  is  repeated,  the  other  will  necessarily  be  re- 
peated also." 

Memory  and  associative  memory,  like  other  nervous 
phenomena,  make  their  appearance  as  soon  as  the  nervous 
system  attains  to  a  certain  complexity.  Loeb  says  that 
"it  can  be  shown  that  infusoria,  coelenterates,  and  worms 
do  not  possess  a  trace  of  associative  memory  and  there- 
fore no  intelligence.  .  .  .  Certain  insects  have  been 
shown  to  display  intelligence — wasps,  etc.  Yerkes  has 
shown  that  certain  crustaceans  possess  a  glimmering  of  it. 

The   greater   the   complexity   of  the   central   nervous 


THE    HIGHER    ORGANISMS 


173 


system  becomes,  the  more  extensive  are  the  associations 
and  memories,  and  the  greater  the  intelligence. 


Fio.  66.  — Scheme  of  reflex  nervous  action.     Relationship  of  cells  and  fibres 
of  brain  and  spinal  cord. 


If  we  found  it  difficult  to  believe  that  the  animal  en- 
gaged in  a  complicated  instinctive  action  did  not  know 
what  it  was  doing,  it  becomes  much  more  difficult  to  be- 
lieve that  an  animal  that  remembers  the  results  of  its 


174  BIOLOGY:  GENERAL  AND  MEDICAL 

former  attempts  and  modifies  its  behavior  so  as  to  over- 
come the  difficulties  in  the  way  of  performance,  and  finally 
wins  its  way  to  success  through  improvement  of  the  method, 
does  not  know  what  it  is  doing. 

Knowledge  of  what  one  is  doing  is  consciousness,  and 
we  next  pause  to  inquire  how  it  fits  in  with  the  general 
views  of  the  functions  of  the  nervous  system  already  ad- 
vanced. 

Consciousness. — As  it  was  impossible  to  find  the  thresh- 
old of  the  various  manifestations  of  nervous  energy  that 
have  been  considered,  so  it  is  impossible  to  say  at  what 
point  in  the  development  of  the  nervous  system  conscious- 
ness makes  its  appearance.  Shall  it  be  imagined  that  the 
protozoa,  in  whose  protoplasm  we  find  the  foreshadowings 
of  most  specialized  functions,  are  consciousness?  Surely 
not.  It  is  only  necessary  to  reflect  upon  the  succession  of 
reflex  activities  that  go  on  inside  of  us  all  the  time,  govern- 
ing the  activities  of  our  internal  organs,  maintaining  and 
timing  the  respiratory  movements,  the  cardiac  movements, 
the  digestive  activities,  etc.,  to  realize  that  they  take 
place  entirely  independently  of  our  consciousness.  Muscle 
reflexes  maintain  our  balance  and  regulate  our  posture 
every  moment  without  consciousness  on  our  part.  De- 
fensive reflexes  now  and  then  surprise  us  as  we  become 
conscious  of  them  after  the  acts  themselves  have  been 
performed.  The  frog  with  its  brain  destroyed  leaps  from 
the  hot  water  into  which  it  is  thrown  or  wipes  away  the 
acid  applied  to  one  leg  by  skilful  movements  of  the  other 
leg  without  the  least  possibility  of  knowing  what  it  is 
doing.  The  brainless  pigeon  thrown  into  the  air  flies  to 
a  convenient  perch  and  comes  to  rest  without  knowing 
what  has  happened. 

Even  in  our  own  normal  bodies  the  most  complex 
activities  sometimes  result  from  automatic  activities 
without  consciousness,  as  when  one  walks  and  talks  in 
his  sleep,  or  continues  to  play  complicated  music  involving 
difficult  technic  while  looking  away  from  the  piano  and 
talking  about  something  else. 


THE   HIGHER   ORGANISMS  175 

If  all  these  acts  can  be  performed  without  conscious- 
ness, what  is  consciousness,  how  important  is  it,  and 
where  in  the  development  of  the  nervous  system  does  it 
appear? 

Parmelee  describes  consciousness  as  "a  complex  process 
made  up  of  feelings  and  ideas  which  are  unified  by  the 
sense  of  personality,  which  may  begin  as  a  vague  feeling, 
but  which  become,  in  the  course  of  time,  a  clear-cut  idea." 

It  would  seem  probable  that  the  "vague  feelings" 
might  begin  low  in  the  animal  scale,  but  the  "clear-cut 
idea"  can  only  be  present  when  the  nervous  system  reaches 
its  maximum  development. 

Herrick  explains  that  "consciousness  is  a  functional 
phase  of  the  more  complex  mechanism  of  those  higher 
non-stereotyped  actions  for  which  the  reflex  machinery  is 
inadequate,  in  much  the  same  way  that  the  tropisms  of 
Paramcecium  and  the  sucking  reflex  of  an  infant  are 
functional  phases  of  the  simple  inborn  neuromuscular 
mechanisms  of  these  organisms." 

Consciousness  is  best  recognized  in  connection  with 
volition  which  "manifests  itself  when  the  idea  of  an  act, 
which  has  grown  out  of  memory  images  of  the  act,  and 
other  contents  of  the  memory,  influence  behavior." 

Consciousness  can,  therefore,  be  thought  to  exist  when- 
ever behavior  is  influenced  by  ideas  or  feelings.  Like 
the  other  phenomena  of  the  central  nervous  system  it  is 
progressive.  It  begins  as  a  mere  "glimmering,"  as  in 
Crustacea  and  insects,  and  is  amplified  and  extended  as 
the  structure  of  the  brain  improves.  It  is  not  a  new  and 
suddenly  appearing  quality  peculiar  to  the  highest  organ- 
isms only. 

By  slow  degrees  consciousness  evolves  into  self-con- 
sciousness. Parmelee  explains  that  this  "appears  as  a 
result  of  the  process  of  integration  which  gives  diverse 
psychic  elements  a  certain  unity." 

It  is  doubtful  whether  self-consciousness  exists  apart 
from  the  very  highest  animals.  Indeed,  it  seems  to  vary 
so  greatly  among  the  races  of  men  as  to  lead  to  the  suspi- 


176  BIOLOGY:  GENERAL  AND  MEDICAL 

cion  that  it  is  a  human  quality  not  to  be  found  among 
lower  animals.  This  is  quite  in  keeping  with  the  view  of 
Parmelee,  who  says,  "self-consciousness  cannot  develop 
very  far  before  it  becomes  an  idea,  and  its  evolution  as  an 
idea  has  been  principally  if  not  entirely  among  men." 

Mind. — Mind  may  be  regarded  as  a  general  term  com- 
prehending all  of  the  phenomenon  centralizing  in  that 
part  of  the  nervous  system  known  as  the  cerebrum.  It 
comprises  intelligence,  consciousness,  and  self-conscious- 
ness. When  these  begin,  mind  begins;  as  they  develop,  it 
develops.  It  is  only  when  mind  reaches  a  certain  develop- 
ment that  reason  appears.  Exactly  at  what  point  is  not 
yet  determined.  Efforts  to  show  that  animals  lower  than 
anthropoid  apes  can  reason  seem  to  have  failed.  In  the 
anthropoid  apes  the  faculty  of  reasoning  is  undoubtedly 
present,  though  not  to  a  degree  that  is  able  to  cope  with 
more  than  the  most  simple  problems.  But  this  is  not 
remarkable,  as  the  faculty  of  reasoning  among  men  varies 
greatly  according  to  race  and  intellectual — i.  e.,  cerebral 
— development,  as  well  as  according  to  the  possession  of 
the  means  of  reasoning — language,  knowledge,  etc.  The 
most  elaborately  developed  brain  of  the  man  best  trained 
in  language  and  possessed  of  the  greatest  sum  of  human 
knowledge  seems  best  able  to  reason  correctly. 

It  is  difficult  for  one  not  acquainted  with  the  details 
of  nervous  structure  to  conceive  of  the  complexity  of 
nervous  activity  arising  in  the  course  of  a  single  and 
apparently  trifling  act.  A  few  moments  ago,  having  clip- 
ped some  papers,  you  carelessly  laid  the  sharp  pointed 
scissors  on  the  desk  where  a  little  later  they  were  covered 
with  some  papers  and  a  blotter.  Moving  your  hand  to 
brush  the  accumulation  aside,  you  felt  a  sharp  prick, 
found  your  hand  involuntarily  drawn  away,  and  recog- 
nized that  you  had  unexpectedly  injured  yourself.  The 
point  of  the  scissors  touching  the  skin  stimulated  a 
peripheral  nerve  ending  in  so  violent  fashion  that  a 
double  excitation  followed,  almost  simultaneously  regis- 
tering pain  in  the  receptive  centres  of  the  brain  and 


THE   HIGHER   ORGANISMS  177 

stimulating  a  motor  centre  in  the  spinal  cord  by  which 
an  impulse  was  sent  out  to  the  muscles  of  the  arm 
which  was  quickly  drawn  away  by  their  contraction. 
In  the  meantime,  the  metal  impression  is  being  rapidly 
passed  about  from  cell  group  to  cell  group  until  it  arrives 
at  a  group  of  cells  formerly  stimulated  in  the  same 
manner  which  now  feebly  revive  the  sensation,  as  one 
produced  by  a  sharp  object,  and  then  to  another  group 
of  cells  which  recall  the  scissors,  and  from  these  to  others 
by  which  you  become  reminded  of  all  that  was  done  a 
short  time  before  and  that  you  had  left  the  scissors  on 
the  table.  The  revived  memories  in  these  nerve  cells 
thus  define  themselves  as  thoughts,  appearing  at  first 
with  such  rapidity  that  they  were  very  indistinct,  but 
becoming  more  and  more  clear  as  time  is  allowed  for  each 
to  arise,  and  as  attention  is  directed  toward  it.  Indeed, 
if  no  means  of  interrupting  the  course  of  nervous  dis- 
charges arising  in  the  brain  in  this  manner  is  adopted, 
and  if  no  new  and  lively  impression  is  received,  the  mem- 
ories aroused  in  one  group  of  cells  after  another  continue 
along  in  an  orderly  sequence,  as,  for  example,  self- 
reproach  for  the  carelessness  shown  in  leaving  the  scissors 
on  the  table,  the  advantage  of  blunt  over  sharp  scissors 
under  such  circumstances,  the  maternal  admonition  to 
order  and  carefulness  often  expressed  in  early  days,  and 
so  on  and  on. 

If  what  may  be  regarded  as  a  relatively  simple  act  is 
attended  with  such  complex  and  correlated  nervous 
activity,  how  much  greater  it  becomes  when  some 
relatively  complex  act  is  considered.  Thus  from  the 
garden  comes  a  stimulus  that  excites  the  nerve  endings 
in  the  mucous  membrane  of  the  nose  and  is  transmitted 
to  the  appropriate  cells  of  the  brain  which  receive  the 
impression  as  "perfume  of  rose.11  When  this  impression 
has  been  properly  registered,  you  turn,  look  out  of  the 
window  and  see — receive  a  visual  impression  of — a  rose. 
How  beautiful!  you  must  go  and  pick  it.  Impulses 
now  descend  from  the  brain  to  the  spinal  cord,  by  which 

12 


178  BIOLOGY:  GENERAL  AND  MEDICAL 

a  succession  of  semi-automatic  movements  is  initiated. 
Thus,  you  first  rise  from  your  chair,  then  put  on  your 
hat,  then  open  the  door,  then  walk  through  the  garden, 
and  then  pluck  the  rose  which  you  place  in  your  button- 
hole. Each  of  these  acts  is  semi-automatic  because  it 
can  be  performed  semi-consciously,  that  is,  without 
special  attention,  having  so  often  been  performed  before 
as  to  have  become  thoroughly  coordinated.  Who 
thinks  what  he  is  doing  as  he  walks  along  the  street? 
The  action  is  thoroughly  coordinated  and  purely  auto- 
matic, but  it  is  not  so  with  the  infant  learning  to  walk, 
and  it  is  not  so  with  some  new  method  of  progress,  as, 
for  example,  walking  upon  stilts  or  gliding  upon  skates. 
An  adult  learning  such  tricks  is  painfully  conscious  of 
the  lack  of  the  proper  coordination  for  the  required 
movements. 

Presumably  the  automatic  movement  has  the  same 
foundation  as  the  thought.  Each  is  a  cell  memory.  A 
cell  or  group  or  cells,  having  been  once  impressed,  recalls 
the  same  impression  and  passes  it  around  in  the  same 
manner,  producing  definite  impressions  upon  group  after 
group.  The  act  of  walking  is  not  simple;  the  move- 
ments of  the  limbs  in  balancing  the  heavy  body  as  its 
centre  of  gravity  is  alternately  disturbed  and  recovered, 
is  extremely  complicated,  and  necessitates  the  combined 
efforts  of  many  muscles  brought  into  action  singly  or  in 
combination  in  orderly  sequence.  Yet  this  can  be 
achieved  without  conscious  thought,  because  through 
long  practice  the  cells  remember  the  lessons  they  have 
learned  and  carry  them  through  without  a  mistake. 
How  complicated  is  the  performance  of  a  fine  pianist! 
Does  he  know  each  note  struck?  Not  at  all;  the  whole 
is  a  series  of  wonderfully  well-coordinated,  highly  com- 
plex, automatic  acts  resulting  from  the  precise  activity  of 
well-trained  nerve  cells  whose  memories  do  not  fail. 
How  difficult  to  learn  the  piano  where  the  eye  reading 
the  notes  and  signs  and  the  fingers  interpreting  them 
must  work  in  harmony!  With  what  tears  and  pains 
does  the  child  learn  to  drum  some  simple  composition! 


THE   HIGHER   ORGANISMS  179 

Thus,  a  consideration  of  the  functions  of  the  nervous 
system  inevitably  brings  us  to  psychology,  and  we  are 
tempted  to  inquire  whether  there  is  any  essential  differ- 
ence between  such  motor  automatism  with  its  coordi- 
nated movements  and  the  psychic  movements  we  know 
as  thoughts.  The  answer  should  be  no.  There  are  no 
differences  other  than  may  be  accounted  for  by  the 
materials  and  the  mechanism.  Thought  seems  to  be  a 
succession  'of  nerve  transmissions  following  one  another 
in  endless  number  and  in  orderly  sequence,  having  their 
source  in  an  external  impression.  Once  set  in  motion, 
the  stimulus  passes  on  and  on,  the  memory  of  each  cell 
reviving  some  other  related  memory  in  another  cell. 
Experience  shows  that  these  memories  arise  simulta- 
neously in  many  cells,  though  the  more  lively  are  usually 
developed  to  the  exclusion  of  the  others.  Each  thought 
has  its  beginning  in  some  external  impression.  Those 
who  doubt  this  may  amuse  themselves  by  endeavoring 
to  create  something  in  thought. 

But  the  higher  animals  not  only  live  in  adjustment 
to  the  external  world;  they  have  internal  organs  whose 
functions  are  indispensable,  and  upon  whose  coordi- 
nated activities  the  life  of  the  whole  body  depends.  For 
these  there  must  be  governing  mechanisms,  and  chief 
among  them  we  again  find  the  nervous  system.  Here, 
however,  automaticity  of  operation  and  properly  cor- 
related action  are  the  chief  requirements.  These  func- 
tions progress  without  intellection.  The  nervous  ar- 
rangements by  which  this  work  is  done,  therefore,  form 
an  almost  independent  system,  the  sympathetic  system,  by 
which  the  organs  are  automatically  innervated.  Thus, 
the  heart  beats  continually — automatically — though  it 
communicates  with  the  central  nervous  system  through 
the  vagus  nerves  and  with  the  sympathetic  system  through 
its  cardiac  branches,  and  is,  therefore,  impressed  by  general 
psychic  conditions.  It  is  difficult  to  trace  the  inception 
of  this  part  of  the  nervous  system,  as  automatic  action, 
such  as  it  supplies  to  the  organs  of  the  higher  animals, 


180  BIOLOGY:  GENERAL  AND  MEDICAL 

is  one  of  the  first  functions  to  make  its  appearance  in  the 
lower  forms  of  life.  No  separation  of  the  two  branches  of 
the  nervous  system  into  sensory-motor  and  sympathetic 
systems  can  be  made  out  in  animals  lower  than  the  arthro- 
pods, with  the  single  exception  of  the  leeches. 

REFERENCES. 

THOMA.S   H.    HUXLEY:    "Anatomy   of   Invertebrated    Animals," 

N.    Y.,    1885.     "Anatomy    of    Vertebrated    Animals." 

N.  Y.,  1886. 
W.   T.   SEDGWICK   AND   E.    B.    WILSON:     "An   Introduction   to 

General  Biology,"  N.  Y.,  1895. 
A.  S.  PACKARD:     "Zoology"  N.  Y.,  1895. 
A.    T.     MASTERMAN:     "Elementary     Text-book     of     Zoology," 

Edinburgh,  1902. 

T.  W.  GALLOWAY:     "First  Course  in  Zoology,"  Phila.,  1906. 
ANTON    KERNER    VON    MARILAUN:     "The    Natural    History    of 

Plants."     Translated  by  F.  W.  Oliver,  N.  Y.,  1894. 
E.  STRASBURGER,  F.  NOLL,  H.  SCHENCK,  AND  G.  KARSTEN:     "A 

Text-book  of  Botany,"  translated  by  W.  H.  Lang,  N.  Y. 

and  London,  1908. 
J.  Y.  BERGEN  AND  B.  M.  DAVIS:     "Principles  of  Botany,"  N.  Y., 

1906. 
T.  J.  PARKER:     "Lessons  in  Elementary  Biology,"  London,  1893. 


CHAPTER  VIII. 
REPRODUCTION. 

The  most  simple  living  organisms  multiply  by  division 
with  or  without  karyokinetic  changes  of  the  nucleus. 
In  most  cases  such  division  is  binary;  that  is,  results  in 
two  of  the  same  general  kind;  but  in  special  cases, 
sporulation,  it  is  multiple  and  gives  rise  to  a  varying 
number  of  offspring  that  differ  from  the  parent  in  being 
much  smaller  and  also  in  certain  cases  in  being  obliged 
to  pass  through  a  succession  of  changes  before  reaching 
maturity  and  parental  resemblance.  Other  lowly  organ- 
isms reproduce  by  a  different  method  known  as  gemma- 
tion or  budding.  In  these  forms,  of  which  the  yeasts 
will  serve  as  examples,  the  adult  cell  throws  out  a  minute 
bud  or  excrescence  which  grows  larger  and  larger  and 
more  and  more  like  the  parent.  As  the  bud  grows, 
the  nucleus  of  the  cell  divides  by  some  modification  of 
the  karyokinetic  process,  one-half  being  retained  in 
the  parent  cell,  the  other  half  passing  into  the  bud  which 
eventually  separates  as  a  new  individual. 

Inasmuch  as  both  division  and  gemmation  result  in 
the  multiplication  of  a  single  cell,  these  methods  are 
described  as  asexual  or  monogenetic  reproduction. 

The  ability  of  any  cell  to  multiply  depends  upon  its 
inherited  impulses  and  upon  its  conditions  of  life. 
Thus,  though  multiplication  is  continually  in  progress 
among  the  unicellular  forms  of  life,  and  characterizes 
the  great  body  of  cells  among  the  metaphyta  or  higher 
plants,  it  is  found  among  the  metazoa  or  higher  animals 
only  during  the  period  of  growth.  When  maturity — 
i.e.,  the  full  size  and  complete  differentiation — has  been 
181 


182 


BIOLOGY:  GENERAL  AND  MEDICAL 


reached,  most  of  the  higher  animals  cease  to  grow  and 
most  of  their  cells  to  multiply. 

Unrestricted  powers  of  multiplication  are  indispensable 


FIG.  67. — Sporulation.  Developing  stages  of  Coccidium  oviforme.  1,  Young 
intracellular  organism,  somewhat  elongated;  2,  an  epithelial  cell  containing 
two  young  organisms,  undifferentiated  in  type;  against  the  surface  of  the 
larger  organism  is  a  disc  of  eosin-staining  substance;  3,  4,  5,  stages  in  the 
development  of  theschizont;  6,  merozoites  as  seen  in  stained  smears;  7,  merozoites 
arranged  in  a  rosette,  about  the  restkorper.  (From  drawings  made  with  camera 
lucida,  Zeiss  comp.,  ocular  No.  4,  1/12  homogeneous  immersion.)  (After 
Tysser.) 


Fio.  68. — Budding  yeast  cells  (Saccharomyces  cerevisiae),  showing  four 
successive  stages  manifested  by  the  nuclear  apparatus.  The  large  pale  sphere 
is  the  nuclear  vacuole,  the  small  dark  sphere  the  nucleolus. 


to  the  lowly  forms  of  life  as  the  sole  means  of  perpetu- 
ating their  kind.  But  among  the  metazoan  animals 
where  the  cells  are  but  units  in  a  complex  structure, 
little  is  to  be  gained  by  the  indefinite  growth  of  the 


REPRODUCTION 


183 


individual  through  the  unrestricted  multiplication  of 
his  component  cells,  and  the  perpetuation  of  the  kind 
must  be  secured  through  discontinuous  growth — i.e.  ~a 


off- 


FIG.  69. — Conjugation  of  Paramoecium  caudatum.  [A-C,  after  R.  Hertwig; 
D-K,  after  Maupas.]  (The  macronuclei  dotted  in  all  the  figures.)  A,  micro- 
nuclei  preparing  for  their  first  division;  B,  second  division;  C,  third  division; 
three  polar  bodies  or  "corpuscules  de  re'but,"  and  one  dividing  germ-nucleus 
in  each  animal;  D,  interchange  of  the  germ-nuclei;  E,  the  same,  enlarged; 
F,  fusion  of  the  germ-nuclei;  G,  the  same  enlarged;  H,  cleavage-nucleus  (c) 
preparing  for  the  first  division;  I,  the  cleavage-nucleus  has  divided  twice;  J,  after 
three  divisions  of  the  cleavage-nucleus;  the  macronucleus  is  breaking  up;  K,  four 
of  the  nuclei  enlarging  to  form  the  new  macronuclei. 

spring.  In  such  animals  it  is  not  through  the  somatic 
or  general  body  cells,  but  through  a  certain  few  germ- 
inal or  reproductive  cells,  early  set  aside  and  highly 
specialized  for  this  purpose,  that  this  is  made  possible. 


184 


BIOLOGY:  GENERAL  AND  MEDICAL 


Such  cells  may  be  given  off  at  any  time  after  the  parent 
attains  to  a  certain  development,  after  the  completion 
of  which,  the  purpose  of  life  having  been  fulfilled,  it 
usually  becomes  decadent. 

Among  the  most  lowly  living  things,  both  animal  and 
vegetable,  growth  is  regularly  followed  by  fission  or 
budding.  But  as  the  higher  organisms  are  reached,  and 
the  foreshadowings  of  organs  appear,  multiplication 
becomes  complicated  by  conditions  not  clearly  under- 
stood, though  no  doubt  of  deep  biological  significance. 


(f/ 


FIG.  70. — Volvox  globator,  showing  the  uniform  and  unspecialized  character 
of  the  cell  structure.  The  large  cells,  o  and  s,  the  oocytes  and  spermatocytes, 
are  the  reproductive  cells  and  alone  show  specialization.  (Lang.) 

Thus,  when  paramrecia  are  kept  in  small  aquaria  under 
what  seem  to  be  appropriate  conditions,  multiplication 
by  fission  proceeds  for  a  certain  time,  after  which  the 
organisms  appear  to  languish,  may  cease  to  multiply, 
become  inactive,  and  tend  to  die  out.  If,  however,  they 
are  frequently  transplanted  to  fresh  sterilized  hay-infu- 
sions, they  continue  to  live  and  multiply  for  an  almost 
indefinite  period.  Thus,  Woodruff  has  been  able  to  keep 
them  alive  and  in  a  state  of  healthy  multiplication  for 
2000  generations.  Ordinarily,  however,  when  thus  kept 
they  die  and  disappear. 

But  Maupas  observed  that  if  at  the  period  of  decline, 


REPRODUCTION  185 

when  the  organisms  were  becoming  inactive,  the  contents 
of  two  aquaria  were  poured  together,  a  phenomenon 
known  as  conjugation  quickly  takes  place.  Two  organ- 
isms, presumably  one  from  each  stock,  become  attached 
to  one  another  by  what  might  be  called  their  ventral 
surfaces — i.e.,  the  surfaces  containing  the  oral  apertures — 
undergo  a  partial  fusion  of  the  surface,  adjust  their  cilia 
so  that  they  move  synchronously,  and  remain  united  for 
some  time,  during  which  a  complicated  interchange  of 
cellular  and  nuclear  substance  takes  place. 


FIG.    71. — Epistylus    umbellaria.     Showing    conjugating    cells.     (After    Graeff 
from  R.  Hertwig.) 

When  this  interchange  is  satisfied,  the  conjoined  indi- 
viduals separate  and  each  again  begins  to  multiply  by 
fission  as  though  the  virility  of  their  respective  strains 
had  never  diminished. 

From  this  experiment  we  learn  that  material  derived 
from  two  individuals  that  have  for  some  time  been 
accustomed  to  a  somewhat  different  environment,  for 
some  unknown  reason  affords  the  cell  greater  vigor  than 
that  exclusively  its  own. 

In  many  cases  conjugation  appears  to  be  an  occasional 
phenomenon  in  which  there  are  no  essential  differences 
between  the  cells  participating;  in  a  far  greater  number  of 


186  BIOLOGY:  GENERAL  AND  MEDICAL 

cases  there  are  such  constant  differences  between  the  con- 
joining cells  that  it  is  possible  to  separate  them  into  male 
and  female  elements,  or  gametes.  The  development  of 
sexual  differences  may  or  may  not  be  incompatible  with 
the  continuance  of  reproduction  by  fission,  though  in 
general  it  marks  the  end  of  the  asexual,  or  monogenetic, 
and  the  beginning  of  the  sexual,  or  digenetic,  mode  of  re- 
production. 

As  in  the  most  simple  forms  of  life,  there  are  no  visible, 
and  probably  no  theoretical  differences  between  the 
occasionally  conjoining  cells,  so  we  find  that  among  the 
primitive  forms  in  which  conjugation  is  constant,  and 
special  cells  are  generated  for  the  purpose,  the  relation 
of  these  cells  to  one  another  is  so  close  that  they  not  in- 
frequently descend  from  the  same  parent  cell.  Thus 
a  spore  of  the  malarial  parasite  having  attained  maturity, 
divides  into  a  considerable  number  of  spores  which 
develop  and  again  divide  until  eventually  through  this 
asexual  mode  of  reproduction  a  great  number  of  the 
parasites  is  produced,  all  the  progeny  of  a  single  cell. 
By  and  by,  however,  a  time  comes  when  the  mature 
cells  cease  to  sporulate  as  usual,  and  develop  into  mature 
sexual  forms,  gametes,  with  which  further  development 
rarely  occurs  unless  conjugation  be  permitted. 

If,  in  this  case,  it  should  be  argued  that  there  is  no 
certainty  that  the  conjoining  male  and  female  elements 
are  derived  from  the  same  parent  because  the  patient 
may  have  a  multiple  infection,  examples  taken  from  the 
primitive  vegetable  world  may  be  given  to  prove  the 
case. 

In  the  reproduction  of  Eurotium  repens  both  the 
asexual  and  sexual  methods  may  be  observed.  The 
former,  which  is  accomplished  through  spores,  may  be 
looked  upon  as  a  kind  of  fission;  the  latter,  after  a  definite 
conjugation,  results  in  the  formation  of  a  peculiar  peri- 
threcium  in  which  a  smaller  number  of  ascospores  is 
developed.  The  formation  of  the  perithrecia  is  not 
easy  to  follow.  As  described  by  de  Bary,  "  they  begin 
in  the  form  of  tender  branches  which  at  the  termination 


REPRODUCTION 


187 


of  their  longitudinal  growth  begin  to  twine  their  free 
ends  in  a  spiral  of  four  or  six  turns;  the  threads  of  the 
spiral  gradually  approach  nearer  together,  until  finally 
they  are  brought  into  contact  so  that  the  entire  end  of 
the  filament  takes  the  form  of  a  helix  (the  ascogonium) . 
There  then  grow  from  the  lowest  part  of  the  helix  two 
or  more  small  branches,  which  cling  closely  to  the  spiral. 
One  of  these  quickly  outstrips  the  others  in  growth; 


PIG.  72. — Development  of  Eurotium  repens.  A,  Small  part  of  a  mycele  with  the 
conidiophore,  c,  and  young  ascogones,  as;  B,  the  spiral  ascogone,  as,  with  the 
antheridial  branch,  p;  C,  the  same  with  the  filaments  beginning  to  grow  round 
it  to  form  the  wall  of  the  sporocarp;  D,  a  sporocarp  seen  from  without;  E,  F, 
young  sporocarp  in  optical  longitudinal  section;  w,  parietal  cells;  /,  the  filling 
tissue  (pseudoparenchymatous) ;  as,  the  ascogone;  G,  an  ascus;  H,  an  ascospore. 
(After  De  Bary). 

its  upper  extremity  reaches  the  uppermost  turn  of  the 
helix  and  fuses  with  it.  The  other  branch  or  branches 
likewise  grow  upward  along  the  spirals,  shoot  out  new 
branches  and  gradually  become  so  interlaced  that  the 


188 


BIOLOGY:  GENERAL  AND  MEDICAL 


spiral  is  finally  surrounded  by  an  unbroken  envelope. 
These  branches  divide  slowly  into  septa  perpendicular 
to  the  surface  and  the  envelope  consists  of  short  angular 
cells  in  which  new  septa  appear  parallel  to  the  surface, 
so  that  the  envelope  thickens  and  is  composed  of  many 
layers.  The  small  sphere  now  formed  is  about  0 . 25  mm. 
in  diameter;  the  outermost  layer  is  yellow,  whilst  the 


FIG.  73. — Ulothrix  zonata.  A,  Young  filament  with  rhizoiil  cell,  r  (X  about 
300);  B,  portion  of  filament  with  escaping  swarm  spores;  C,  single  swarm  spore; 
D,  formation  and  escape  of  gametes;  E,  gametes;  F,  G,  conjugation  of  two 
gametes;  H,  zygote;  J,  zygote.  after  peried  of  rest;  K,  zygote  after  division 
into  swarm  spores.  (After  Dodel-Port.  B-K  X  about  482.) 


inner  layers  remain  soft,  and  later  are  dissolved.  The 
spiral,  after  a  time,  extends  and  throws  out  on  all  sides 
branched  filaments  which  dislodge  the  inner,  layers 
of  the  envelope.  These  branches  finally  take  the  form 
of  an  ascus,  eight  spores  being  formed  in  each.  After 


REPRODUCTION  189 

the  breaking  up  of  the  asci,  the  spores  lie  loose  in  the 
interior  of  the  perilhrecium  and  are  liberated  by  the 
rupture  of  the  fragile  wall  of  the  latter." 

In  this  example  the  conjugation  embraces  two  ele- 
ments not  only  belonging  to  the  same  individual,  but 
also  to  the  same  mycelial  filament. 

Among  the  algae  same  interesting  forms  of  conjuga- 
tion of  elements  derived  from  the  same  parent  may  also 
be  observed.  Thus,  in  Ulothrix  zonata  sexual  and 
asexual  reproduction  may  be  seen.  The  asexual  repro- 
duction is  accomplished  by  swarm  spores  each  of  which 
is  provided  with  four  cilia.  These  are  formed,  so  far 
as  can  be  determined,  by  any  cell  in  the  filament  of  which 
Ulothrix  is  composed,  and  having  escaped  through  an 
opening  in  the  side  of  the  cell  are  immediately  ready 
to  swim  away  and  start  a  new  growth  resembling  the 
parent.  The  sexual  reproduction  is  accomplished 
through  gametes  which  closely  resemble  the  swarm  spores 
except  that  they  are  smaller  and  possess  but  two  fla- 
gella  each.  These  likewise  escape  through  an  opening  in 
the  lateral  wall  of  the  parent  cell,  but  proceed  to  con- 
join, forming  zygotes  or  fertilized  cells,  which  draw  in 
the  flagella,  become  spherical,  surround  themselves  with 
a  cell  wall  and  enter  upon  a  period  of  rest,  after  which 
they  divide  into  a  number  of  swarm  spores  which  in  turn 
escape  from  the  capsule  and  swim  away  to  found  new 
plants.  In  case  the  gametes  are  unable  to  conjugate, 
they  may  behave  like  the  swarm  spores  and  themselves 
found  new  individuals. 

Here,  therefore,  we  find  a  plant  with  such  loose  methods 
of  reproduction  that  even  its  gametes  or  sexual  cells 
may  under  certain  conditions  fail  to  conjoin,  but  carry 
out .  a  parthenogenetic  development — i.e.,  germination 
without  conjugation  or  fertilization. 

Another  interesting  and  instructive  example  is  found 
in  the  self-conjugation  of  Vaucheria  which  might  be 
likened  to  a  form  of  hermaphroditism  or  bisexual  nature 
of  one  individual.  Here  we  have  a  filamentous  alga 
whose  terminal  cell  becomes  specialized  into  a  sporan- 


190 


BIOLOGY:  GENERAL  AND  MEDICAL 


gium  containing  a  zoospore  which  escapes  by  rupturing 
the  apex  of  its  wall  by  means  of  a  rotary  motion.  This 
zoospore  is  large  enough  to  be  seen  with  the  unaided  eye, 
consists  of  mass  of  colorless  protoplasm  containing  nu- 
merous nuclei  and  is  surrounded  on  all  sides  by  cilia,  two 
of  which  are  given  off  opposite  each  nucleus,  and  is  said 
by  Strasburger  to  correspond  to  the  collective  individual 
zoospores  of  an  ordinary  sporangium.  This  zoospore 


PIG.  74. — Sexual  reproduction  of  the  green  felt  (Vaucherid).  A,  Vaucheria 
sessilia;  o,  oogonium;  a,  antheridium;  os,  the  thick-walled  oospore,  and  beside 
it  an  empty  antheridium;  B,  Vaucheria  geminata,  a  short  lateral  branch  develop- 
ing a  cluster  of  oogonia  and  a  later  stage  with  mature  oogonia  o  and  empty 
antheridium  a;  C,  sperms;  D,  germinating  oospore.  (C,  after  Woronin;  D, 
after  Sachs.)  (From  Bergen  and  Davis,  "Principles  of  Botany."  Ginn  &  Co., 
publishers.) 

immediately  develops  into  a  new  plant.  In  addition  to 
this  and  more  important  in  this  argument  is  the  sexual 
reproduction.  From  the  same  cell  of  one  of  the  fila- 
ments two  small  protuberances,  the  oogonium  and  an- 
theridium, containing  the  female  and  male  elements,  re- 
spectively, make  their  appearance  and  soon  develop  as 
short  lateral  branches  which  become  separated  from 
the  rest  of  the  thallus.  It  is  said  that  the  oogonium  at 


EEPRODUCTION 


191 


first  contains  several  nuclei  of  which  all  but  one  subse- 
quently retreat  into  the  main  filament  again  before  the 
septation  is  completed. 

The  oogonium  eventually  forms  a  rounded  mass  with  a 
truncated  conical  projection  of  clear  protoplasm  on  one 
side  where  it  opens.  The  antheridium  which  is  multi- 
nuclear  and  more  or  less  coiled  or  like  a  hook  is 
open  at  the  tip.  Its  contents  first  break  up  into  swarm- 


FIG.  75. — A,  Conjugation  of  Spirogyra  quinina.  B,  Spirogyra  longata;  z, 
zygospore.  C,  cell  of  Spirogyra  jugalis;  k,  nucleus;  ch,  chroma tophores;  p, 
pyrenoid.  (Strasburger,  Noll,  Schenck,  and  Karsten.) 

ing  spermatozoids  which  it  then  discharges  together  with 
its  mucilaginous  contents.  The  spermatozoids  are  very 
small,  each  being  provided  with  a  single  nucleus  and  two 
cilia.  They  collect  about  the  oogonium  into  which  one 
of  them  eventually  penetrates  to  fertilize  it  by  the  fusion 
of  their  nuclei.  The  oogonium  then  becomes  converted 
into  a  resting  oospore  which  eventually  germinates  with 
the  production  of  a  filamentous  thallus.  Here  we  find  a 


192 


BIOLOGY:  GENERAL  AND  MEDICAL 


peculiar  development  of  the  phenomenon  of  conjuga- 
tion by  which  in  a  certain  sense  the  cell  substance  con- 
joins with  itself. 

Another  curious  form  of  conjugation  is  seen  in  Spiro- 
gyra,  where  two  cells  side  by  side  in  the  same  filament 
conjugate  by  the  development  of  coalescing  processes 
which  are  formed  near  their  transverse  wall.  Here 


Flo.  76. — Mucor  mucedo.  Different  stages  in  the  formation  and  germination 
of  the  zygospore:  1,  Two  conjugating  branches  in  contact;  2,  septation  of  the 
conjugating  cells  (a)  from  the  suspensors  (6);  3,  more  advanced  stage  in  the 
development  of  the  conjugating  cells  (a);  4,  ripe  zygospore  (6)  between  the 
suspensors  (a) ;  5,  germinating  zygospore  with  a  germ-tube  bearing  a  sporangium. 
(After  Brefeld.) 


again  there  can  be  no  doubt  but  that  the  conjoining 
cells  are  the  descendants  of  the  same  parent  and  so 
closely  related  as  to  make  the  advantage  of  the  inter- 
change of  substance  of  problematical  value. 

From  such  indistinct  differentiation  of  male  and 
female  substance,  and  from  aberrant  forms  in  which, 
as  in  Ulothrix,  the  gametes  may  either  conjugate  or 


REPRODUCTION  193 

themselves  go  on  to  adult  development,  we  pass  to  forms 
in  which  the  conjoining  cells  must  come  from  different 
individuals.  Thus  in  Mucor  mucedo  the  formation  of 
zygospores  follows  the  conjugation  of  cells  from  different 
colonies,  never  from  the  conjugation  of  cells  from  the 
same  colony.  The  spreading  mycelia  of  this  mould, 
reaching  the  spreading  mycelia  of  another  colony  may 
or  may  not  conjoin  according  to  the  nature  or  conditions 
of  the  two.  Thus  when  carefully  examined  it  is  found 
that  they  can  be  separated  into  two  strains  represented, 
respectively,  by  the  signs  +  and  — .  Colonies  of  the  + 
strain  will  not  conjoin;  colonies  of  the  —  strain  will  not 
conjoin,  but  +  and  —  will  conjugate  and  form  zygospores. 
The  actual  steps  in  zygospore  formation  are  not  difficult 
to  follow.  The  gametes  are  clavate  terminal  enlarge- 
ments of  the  mycelial  threads.  As  they  come  together 
the  tips  flatten  and  soon  a  short  cell,  the  real  conjugat- 
ing cell  or  gamete,  makes  its  appearance  through  the  for- 
mation of  a  line  of  cleavage  appearing  a  short  distance 
from  the  point  of  contact. 

The  original  mycelium  is  now  known  as  a  suspensorium, 
the  conjoined  cells  as  gametes.  There  may  be  one  or 
two  zygospores  according  to  the  subsequent  develop- 
ment of  one  or  both  of  the  gametes.  In  cases  where 
actual  fusion  of  the  gametes  takes  place,  there  can  be 
but  one  zygospore;  where  a  transfer  of  substance  from 
one  to  the  other  occurs  without  fusion,  two  may  form. 
The  zygospores  become  surrounded  with  a  thick  mem- 
brane which,  under  favorable  conditions,  ruptures  to 
permit  the  escape  of  the  growing  hypha.  The  advan- 
tage of  conjugation  in  these  cases  is  not  clear,  for  the 
formation  of  spores  surrounded  by  dense  membranous  en- 
velopes— azygospores — may  occur  without  conjugation. 

Thus  in  examining  the  lowly  forms  of  life,  both  animal 
and  vegetable,  we  are  struck  by  the  growing  tendency 
toward  amphimixis  or  the  digenetic  mode  of  reproduc- 
tion, the  importance  of  which  is  shown  by  the  ingenious 
means  taken  to  bring  it  about,  and  the  final  disappear- 
13 


194  BIOLOGY:  GENERAL  AND  MEDICAL 

ance  of  all  other  forms  of  reproduction  among  the  high- 
est animals  and  plants. 

As  we  have  found  the  reproductive  power  a  charac- 
teristic of  living  substance,  so  we  find  this  power  remain- 
ing among  otherwise  differentiated  multicellular  beings 
long  after  more  simple  forms  have  adopted  sexual  modes 
of  development.  Even  after  the  differentiation  of  so- 
matic and  germinal  cells  has  been  effected,  and  germinal 
cells  of  two  sexes  established,  the  general  tendency  of 
the  cells  toward  reproduction  remains  strong  and  shows 
itself  in  a  variety  of  ways. 

Thus,  among  plants,  long  after  sexual  reproduction 
has  been  established  and  even  in  certain  forms  in  which 
sexual  organs  and  seeds  are  produced,  the  asexual  form 
of  multiplication  continues  and  may  be  the  chief  method 
of  reproduction.  The  garlics,  for  example,  produce  no 
seeds,  but  only  bulbs,  and  the  banana,  though  it  pro- 
duces seeds,  continues  to  grow  chiefly  through  bulbous 
buds.  Other  plants,  Chara,  with  well-developed  sexual 
organs,  produce  but  one,  the  female  sex. 

Among  animals,  although  the  asexual  mode  of  repro- 
duction is  chiefly  confined  to  the  protozoa,  we  find  it 
continuing  among  the  ccelenterates  and  worms,  both  of 
which  multiply  by  gemmation  though  eggs  are  also 
produced. 

In  this  particular  the  hydra  is  more  lowly  than  the 
sponges  which  multiply  exclusively  through  eggs, 
because  it  continues  to  multiply  by  buds  as  well  as  by 
eggs.  The  buds  appear  upon  the  body  wall  as  rounded 
eminences,  which  in  the  course  of  time  become  pro- 
vided with  tentacles,  come  more  and  more  closely  to 
resemble  the  parent,  and  finally  separate  themselves  as 
new  individuals.  In  addition,  however,  certain  of 
the  apparently  undifferentiated  cells  of  the  ectoderm 
increase  locally  in  number  forming  gonads  which  con- 
tain the  germ  cells,  one  group  situated  near  the  tentacles 
being  the  homologue  of  the  testes  of  the  higher  animals, 
the  other  usually  developed  near  the  aboral  end,  forming 


REPRODUCTION 


195 


the  ovary  and  containing  the  eggs.  Spermatozoa  are 
produced  in  large  numbers  by  the  testicular  cells  which 
mature  before  the  ova  (protandrism)  and  are  liberated 
by  rupture  of  the  ectodermal  covering.  One  ovum  is 


Fia.  77. — Colony  of  Obelia  geniculata  (magnified).     (After  Masterman.') 


usually  all  that  matures  in  the  ovary.  It  takes  an 
amoeboid  form  also  escaping  by  rupture  of  the  ectoder- 
mal layer,  protruding  between  its  cells,  in  which  position 
it  is  reached  by  the  spermatozoa,  one  of  which  conjugates 
with  it.  When  thus  fertilized,  it  loses  its  amoeboid 


196 


BIOLOGY:  GENERAL  AND  MEDICAL 


movement,  becomes  encysted  and  remains  dormant  for 
several  weeks  after  falling  from  the  parent. 

The  hydra  is  thus  seen  to  develop  in  itself  both  male 
and  female  sexual  elements,  and  is,  therefore,  a  her- 
maphrodite. 

In  other  ccelenterates  a  somewhat  different  sequence 
of  events  is  observed.  Thus  in  the  small  marine  hy- 
droid  polyp,  known  as  Obelia,  we  find  near  the  base  of 
the  stalk  certain  members  somewhat  resembling  the 
polyps,  but  having  no  tentacles  and  containing  rounded 
bodies  of  small  size — the  sporosacs.  When  ripe  they 


BuycanaL 


FIG.  78. — Lateral  view  of  a  medusa  of  Obelia.     Magnified.     (Ad.  nat.)     (After 
Masterman.) 

rupture  and  permit  the  escape  of  small  modified  polyps, 
not  unlike  the  jelly-fishes  in  general  appearance,  though 
extremely  minute,  and  known  as  medusce.  They  are 
umbrella-shaped,  the  opening  below.  From  the  centre 
there  hangs  a  central  member,  known  as  the  manubrium, 
upon  which  the  mouth  opens  into  a  little  bag,  the  ccelen- 
teron  or  general  digestive  cavity,  which  communicates 
with  four  radiating  canals  which  run  to  the  rim  of  the 
umbrella  where  they  connect  with  a  ring  canal,  opposite 
to  which  there  is  a  sense  organ.  The  structure  is  highly 
specialized,  as  contrasted  with  the  hydroid  parents.  The 


REPRODUCTION  197 

medusse  are  free  and  swim  about  by  opening  and  closing 
the  umbrella,  and  nourish  themselves  like  the  jelly-fishes. 
After  a  certain  duration  of  vegetative  life,  gonads,  or 
reproductive  organs,  make  their  appearance.  These  are, 
however,  different  in  the  different  medusse,  some  pro- 
ducing ova,  others  spermatozoa,  so  that  these  little 
animals  are  dioecius,  or  of  two  sexes.  The  eggs,  being 
discharged,  are  fertilized  by  conjugation  with  spermat- 
ozoa in  the  water.  From  these  eggs  a  larva  is  developed 
which  swims  to  the  bottom  of  the  water,  attaches  itself 
to  some  object,  and  develops  into  a  hydroid  polyp. 
Thus,  in  the  life  history  of  this  little  animal  we  find  two 
stages  which  alternate,  one  of  fixed  hydroid  and  one  of 
free  swimming  medusa  form.  It  is  thus  said  to  exhibit 
metagenesis  or  alternation  of  generation. 

Among  the  sponges  the  sexes  seem  to  be  clearly  sepa- 
rated so  that  the  animals  are  all  dicecius.  The  cells 
which  take  on  the  reproductive  function  are  known  as 
gonocytes  and  are  amoeboid.  The  spermatic  cells 
break  up  into  a  considerable  number  of  spermatozoa 
which  are  liberated  into  the  water.  The  ova  which 
form  in  some  other  sponge  are  also  amoeboid  and  force 
their  way  through  the  entodermal  cells  until  they  pro- 
trude into  one  of  the  inhalant  canals  where  they  await 
the  arrival  of  a  spermatozoon  in  the  water.  When 
conjugation  has  taken  place  and  fertilization  been 
effected,  the  ovum  again  draws  back  into  the  body  of 
the  sponge  and  shortly  undergoes  segmentation  by  the 
formation  of  a  considerable  number  of  equal-sized  divi- 
sions resulting  in  a  blastula  or  hollow  sphere  composed 
of  one  layer  of  cells.  The  cells  of  one  hemisphere  next 
increase  in  number  above  those  of  the  other,  and  acquire 
flagella,  after  which  the  embryo  leaves  the  body  of  its 
parent  and  swims  away.  Soon  the  non-flagellated  cells, 
which  appear  granular,  begin  to  grow  and  invaginate 
the  flagellated  cells  so  that  the  embryo,  being  no  longer 
able  to  swim,  settles  down  upon  some  object  to  which 
it  later  becomes  attached.  An  osculum  opens  at  the 


198 


BIOLOGY:  GENERAL  AND  MEDICAL 


free  side,  pores  soon  open  here  and  there  upon  the  sur- 
face, the  internal  cells  throw  out  cilia  into  the  para- 
gastric  cavity,  and  a  sponge  is  formed. 

The  reproduction  of  the  ferns  is  of  considerable  interest 
because  these  plants  again  show  a  peculiar  form  of  con- 
jugation effected  by  male  and  female  cells  of  common 


FIG.  79. — The  fern  prothallium  and  archegonium.  A,  Stages  in  the  germi- 
nation of  the  spore;  B,  young  prothallium,  showing  first  appearance  of  wedged- 
shaped,  apical  cell  x;  C,  tip  of  prothallium  beginning  to  take  on  the  heart-shaped 
form;  x,  apical  cell;  D,  mature  prothallium,  showing  group  of  archegonia  on  the 
cushion  just  back  of  the  notch,  and  antheridia  further  back;  rh,  rhizoids;  E, 
an  open  archegonium  with  egg  ready  for  fertilization  and  two  sperms  near  the 
entrance  of  the  neck.  (A,  B,  C,  E,  after  Campbell;  D,  after  Schenck.)  (From 
Bergen  and  Davis'  "Principles  of  Botany."  Ginn  &  Co.,  publishers.) 

parentage.  Thus  the  fern  produces  a  large  number  of 
spores  which,  however,  do  not  grow  into  ferns,  but  under 
favorable  conditions  develop  into  a  peculiar  thin,  fleshy, 
heart-shaped  mass,  called  the  prothallium.  This  always 
remains  small,  not  exceeding  one  or  two  centimeters  in 
diameter.  The  greater  part  of  it  is  purely  vegetative, 


REPRODUCTION  199 

but  upon  its  under  surface  there  eventually  appear  two 
groups  of  sexual  organs,  antheridia,  constituting  the  male 
elements  and  producing  spermatozoids;  and  archegonia, 
or  female  elements,  producing  each  a  single  egg  cell  or 
ovum.  Water  is  essential  to  the  further  progress  of 
events,  in  order  that  the  spermatozoids,,  which  swim 
freely,  may  reach  the  archegonia,  to  which  they  are  at- 
tracted through  a  chemotropic  affinity,  that  probably  de- 
pends upon  the  presence  of  malic  acid  in  the  latter. 
Reaching  the  ovum  in  the  archegonium  a  spermatozoid 
conjugates  with  it,  after  which  the  fertilized  ovum  devel- 
ops into  the  asexual  plant  or  fern.  Here  we  find  an  alter- 
nation of  generations,  the  purpose  of  which  seems  to  be 
conjugation,  though  how  such  conjugation  of  two  cells 
directly  traceable  to  a  common  parent  can  be  so  essen- 
tial as  to  merit  so  roundabout  a  method  for  its  accom- 
plishment is  difficult  to  imagine. 

The  reproduction  of  the  higher  plants  shows  many 
interesting  endeavors  to  escape  "  self-f ertilization "  and 
profit  by  the  advantages  of  "cross-fertilization,"  though 
the  latter  can  scarcely  be  said  to  assume  the  importance 
that  is  sfeen  among  animals. 

Thus,  among  the  phanerogams,  or  flowering  plants,  we 
find  many  plants  bearing  flowers  possessing  male  (anthers) 
and  female  (pistil)  organs  that  mature  at  the  same  time, 
so  that  their  fertilization  must  usually  be  affected 
by  their  own  pollen  grains  dropping  directly  upon  the 
stigma,  growing  down  to  the  ovules  in  the  ovary,  con- 
jugating with  and  fertilizing  them.  Indeed,  in  cleistoga- 
mous  flowers,  like  the  peas,  in  which  the  sexual  organs 
are  never  exposed,  it  is  scarcely  possible  for  fertilization 
to  take  place  in  any  other  way.  But  although  this  is 
characteristic  of  a  number  of  plants,  so  many  devices 
are  found  among  flowers  in  general,  for  its  prevention, 
that  it  would  seem  as  though  the  best  interest  of  plant 
life  is  opposed  to  it.  Thus  the  stigma  and  anthers  of 
flowers  do  not  usually  mature  at  the  same  time;  i.e.,  they 
are  subject  to  protandry,  or  the  maturation  of  the  an- 


200  BIOLOGY:  GENERAL  AND  MEDICAL 

thers  before  the  stigma,  when  the  flower  must  be  ferti- 
lized by  pollen  from  other  flowers  younger  than  itself, 
or  by  protogyny,  or  maturation  of  the  stigma  before  the 
anthers,  when  it  must  be  fertilized  by  pollen  from  flowers 
older  than  itself.  In  either  case,  the  conjugating  ele- 
ments must  come  from  different  flowers,  though  these 
may  be  upon  the  same  plant. 

In  other  flowers  an  inequality  in  the  length  and 
position  of  the  stamens  and  pistils  prevents  self-pollination 
and  consequent  self-fertilization. 

Some  plants  develop  two  sets  of  flowers,  male  and 
female,  respectively,  and  other  plants  are  of  distinct 
sexes,  certain  individuals  producing  only  male,  and 
others  only  female  flowers. 

In  a  few  cases  the  pollen  from  a  flower  when  dropped 
upon  the  stigma  of  the  same  flower  fails  to  grow  into  the 
usual  filaments,  or,  doing  so  and  descending  to  the  egg 
cell,  fails  to  conjoin  and  fertilize  it,  or  does  so  only  when 
no  pollen  from  a  different  flower  finds  its  way  to  the 
same  stigma.  In  some  cases,  when  pollen  from  the 
same  flower  and  from  a  different  flower  arrive  at  the 
same  time,  upon  the  same  stigma,  the  exogenous  pollen 
appears  to  outgrow  the  autogenous  pollen  and  more 
quickly  finds  its  way  to  the  egg  cells  and  effects  fertiliza- 
tion. It  is  probably  by  this  means  that  cross-fertiliza- 
tion is  effected  in  those  cases  in  which  the  pollen  from 
various  flowers  with  sexual  organs  maturing  at  the 
same  time  is  freely  distributed  by  the  wind.  The  same 
means  of  securing  the  advantage  of  cross-fertilization 
is  probably  adopted  in  those  cases  in  which  the  flowers 
are  entomophilous  and  visited  by  insects  that  not  only 
sprinkle  pollen  belonging  to  the  flower  upon  its  stigma, 
but  also  bring  it  pollen  from  other  and  remote  flowers. 

Thus  the  phaneroganic  vegetation  displays  a  pro- 
nounced disposition  to  secure  cross-fertilization,  though 
its  importance  seems  to  vary  widely  in  different  cases. 

The  animal  kingdom,  however,  shows  a  much  more 
restricted  sexual  development.  Hermaphroditism,  or  the 


REPRODUCTION 


201 


occurrence  of  both  sexes  in  the  same  individual,  is  almost 
as  much  the  exception  in  the  animal  world  as  it  is  the 
rule  in  the  vegetable  world.  It  might  be  reasonable 


FIG.  80. — Withdrawal  and  deposition  of  pollinia  in  the  flowers  of  an  orchid. 
Flowering  spike  of  the  broad-leaved  helleborine  (Epipactis  latifolia)  upon  which 
a  wasp  (Vespa  austriaca)  is  alighting.  2,  A  flower  of  the  same  seen  from  the 
front;  3,  side  view  of  the  same  flower  with  the  half  of  the  perianth  toward  the 
observer  cut  away;  4,  the  two  pollinia  joined  by  the  sticky  rostellum;  5,  the 
same  flower  being  visited  by  a  wasp,  which  is  licking  honey  and  at  the  same 
time  detaching  with  its  forehead  the  tip  of  the  rostellum  together  with  the  pair 
of  pollinia;  6,  the  wasp  leaving  the  flower  with  the  pollinia  cemented  to  its  head, 
the  pollinia  are  erect;  7,  the  wasp  visiting  another  flower  and  pressing  its  fore- 
head with  the  pollinia  (which  in  the  meantime  have  bent  down)  against  the 
stigma.  (Kerner  and  Oliver.) 


to  regard  this  difference  as  referable  to  the  prevailing 
restriction  of  movement  among  plants,  as  contrasted 
with  the  freedom  of  movement  among  animals,  making  it 
correspondingly  difficult  or  easy  for  different  individuals 


202  BIOLOGY:  GENERAL  AND  MEDICAL 

to  reach  one  another  for  reproductive  purposes.  And 
we  see  among  the  hermaphrodite  animals  the  same 
repugnance  to  self-fertilization  where  it  is  not  made 
essential  by  the  peculiar  conditions  of  life.  Thus  of 
hermaphroditic  animals  we  find  self-fertilization  prac- 
tised by  the  parasitic  worms  whose  existence  is  so 
precarious  that  it  is  exceptional  to  find  more  than  one  in 
the  same  host,  but  not  among  the  earthworms  or  snails 
that  are  free  and  independent. 

The  higher  we  ascend  in  the  animal  kingdom,  the 
more  emphatic  is  the  demand  for  conjugation  of  gametes 
from  separate  individuals  of  opposite  sexes.  Not  only 
does  sexual  differentiation  take  place  low  down  in  the 
scale  of  life,  but  the  ability  of  the  germinal  cells  to  undergo 
parthenogenetic  development,  or  to  develop  without 
fertilization,  also  quickly  disappears,  not  being  observed 
among  animals  higher  than  the  arthropods. 

Parthenogenetic  development  seems  to  take  place  only  in 
female  germinal  cells  in  which  there  is  no  reduction  of  chro- 
mosomes (see  below)  and  in  which  there  is  no  conjugation 
with  male  germinal  cells.  It  must  not  be  confused  with 
hermaphroditism.  It  occurs  notably  in  certain  rotifers,  the 
males  of  which  are  not  known,  if  they  exist  at  all;  in  cer- 
tain Branchiopods  and  Astracods,  in  Daphnids  during 
summer;  among  Aphides  or  plant-lice,  and  among  certain 
bees  (social  bees).  It  is  difficult  to  explain,  but  Owen,  as 
early  as  1849,  suggested  that  "not  all  of  the  progeny  of 
the  primarily  impregnated  germ-cell  are  required  for  the 
formation  of  the  body  in  all  animals;  certain  of  the 
derivative  germ-cells  may  remain  unchanged  and  become 
included  in  that  body  which  has  been  composed  of  their 
metamorphosed  and  diversely  combined  or  confluent 
brethren;  so  included,  any  derivative  germ-cell,  or  the 
nucleus  of  such,  may  begin  and  repeat  the  same  proc- 
esses of  growth  by  imbibition,  and  of  propagation  by 
spontaneous  fission,  as  those  to  which  itself  owed  its 
origin."  It  is  to  Owen  that  we  owe  the  word  "partheno- 
genesis." 


REPRODUCTION  203 

Development  by  gemmation  also  disappears  very  early, 
so  that  there  is  nothing  in  the  higher  animal  organisms 
that  is  in  the  least  degree  comparable  to  the  multiplication 
by  budding  so  prevalent  among  the  higher  plants.  Re- 
production among  animals,  therefore,  soon  narrows  itself 
down  to  the  formation  of  gametes,  produced  by  sexually 
different  individuals,  the  conjugation  of  these  gametes 
and  the  formation  of  a  zygote  or  fertilized  cell  which  grows 
into  an  embryo,  which  after  certain  metamorphoses  de- 
velops into  a  sexual  individual  resembling  one  or  the  other 
parent. 

There  is  an  early  differentiation  of  the  cells  of  animals 
into  those  known  as  somatic,  of  which  the  body  proper 
consists,  and  those  known  as  germinal  whose  office  is 
solely  reproductive.  The  latter  are  contained  in  the 
gonads  or  sexual  organs  until  the  organism  to  which 
they  belong  becomes  sexually  mature,  when  they  them- 
selves undergo  certain  essential  changes  preparative  to 
the  fertilization  that  initiates  the  reproductive  process. 
The  function  of  reproduction,  among  the  higher  animals, 
usually  develops  at  the  time  of  maturity  or  complete 
physical  perfection.  There  are,  however,  exceptions,  as  in ' 
the  case  of  the  axolotl  (larva  of  Amblystoma  mexicanum), 
a  batrachian,  which  is  capable  of  sexual  reproduction  in 
the  larval  stage. 

Another  illustration  of  parthenogenetic  development 
and  exception  to  the  customary  modes  of  reproduction 
in  certain  insects  (Cecidomyia)  was  discovered  by  Wagner 
in  1861.  Within  the  bodies  of  the  larvae  of  these  insects, 
daughter  larvae,  exactly  like  the  parent,  develop  and  sub- 
sequently creep  out.  These  parthenogenetic  larvae  develop 
similar  broods,  and  these  others,  throughout  the  entire 
autumn,  winter,  and  spring  months.  With  the  last  gen- 
eration, at  the  beginning  of  summer,  this  mode  of  repro- 
duction ceases  and  the  larvae  pupate,  producing  both  males 
and  females  that  engage  in  ordinary  sexual  reproduction. 
Such  alternation  of  sexual  with  parthenogenetic  repro- 
duction Wagner  called  pcedogenesis. 


204  BIOLOGY:  GENERAL  AND  MEDICAL 

In  vegetables  it  cannot  be  shown  that  the  germinal 
cells  differ  essentially  from  the  somatic  cells.  They 
appear  only  when  the  reproductive  process  is  anticipated, 
attain  to  the  necessary  degree  of  specialization  in  a  few 
generations,  are  characterized  by  a  preparation  for 
fertilization  to  all  intents  and  purposes  identical  with 
that  of  the  animal  cells,  and  then  having  been  fertilized 
by  conjugation  with  another  specially  adapted  cell,  lose 
the  reproductive  quality  and  become  vegetative  in 
character  once  more.  Thus,  in  vegetables  the  repro- 
ductive activity  may  be  said  to  pervade  the  cells  gener- 
ally, while  in  animals  it  is  more  and  more  restricted  to 
the  few  cells  comprising  the  germ  plasm. 

It  is  of  the  utmost  importance  that  the  steps  pre- 
liminary to  sexual  fertilization  be  carefully  followed, 
and  for  this  purpose  it  will  be  necessary  once  more  to 
enter  the  domain  of  cytology. 

Among  both  plants  and  animals  the  germinal  and 
somatic  cells  possess  the  same  number  of  chromosomes, 
yet  the  gametes  contain  but  half  as  many.  This  depends 
upon  a  "reduction  of  chromosomes"  discovered  by  van 
'Beneden,  and  seen  in  the  maturation  of  the  germinal 
cells.  It  is  a  matter  of  much  interest,  and,  as  it  has 
fundamental  bearing  upon  the  problems  of  inheritance 
and  variation,  deserves  much  attention. 

Any  of  the  higher  plants  or  animals  will  be  found  to 
possess  specialized  germinal  cells  set  aside  in  the  gonads 
or  sex  organs  until  sexual  maturity  awakens  them  to  ac- 
tivity. As  there  are  two  sexes,  and  the  sex  organs  and 
their  products  differ,  two  kinds  of  gametes,  the  male,  or 
spermatozoa,  and  the  female,  or  ova,  are  to  be  studied, 
and  two  subjects,  spermatogenesis  and  oogenesis,  appear 
for  investigation. 

Spermatogenesis. — The  germinal  cells  early  set  apart 
in  the  embryonal  gonad — testis — multiply  slowly  during 
the  period  of  growth  and  development,  and  perhaps  more 
rapidly  during  the  period  of  sexual  activity.  No  essential 
difference  in  appearance  separates  them  from  the  somatic 


REPRODUCTION 


205 


cells.  They  possess  the  same  number  of  chromosomes 
and  divide  after  the  usual  karyokinetic  changes — homo- 
type  mitosis. 


Germinal  cell  or  primary  spermatocyte  at  rest. 


Primary  spermatocyte  in  process  of  maturation,  show- 
ing four  chromosomes,  two  (P  and  p)  of  paternal  and 
two  (M  and  m)  of  materna'  origin. 

Conjugation  of  the  chromosomes.  Each  pair  consists 
of  a  paternal  and  a  maternal  chromosome.  The  ap- 
pearance, of  two  tetrad-like  formations,  is  easily  mis- 
interpreted to  mean  that  at  this  stage  the  cell  has 
eight,  or  twice  the  somatic  number  of  chromosomes. 
Or,  if  each  pair  happens  to  be  mistaken  for  a  single 
chromosome,  one-half  of  the  usual  nnmber. 

When  the  conjoined  chromosomes  separate  they  do  not 
undergo  the  longitudinal  splitting  characterizing 
homotype  mitosis,  in  which  one-half  of  each  chromo- 
some goes  to  an  opposite  pole  of  the  cell,  but  whole 
chromosomes  now  move  to  opposite  poles— hetero- 
type  mitosis. 

Thus  arise  the  secondary  spennatocytes,  each  with  two 
or  one-half — the  reduced — number  of  chromosomes. 

From  these  secondary  spermatocytes  the  spermatozoa 
or  male  gametes  are  formed  by  homotype  mitosis, 
that  is,  the  chromosomes  arrange  themselves  equa- 
torially,  divide  longitudinally,  and  one-half  of  each 
goes  to  each  pole  of  the  cell. 

The  process  eventuates  in  mammals  in  four  gametes 
or  spermatozoa  of  equal  value,  each  with  the  reduced 
number — two — of  chromosomes. 


By  subsequent  modifications  of  shape  these  become  the 
well-recognized  spermatozoa. 


Fis.  81. — Diagram  explaining  the  maturation  of  "the  male  germinal  cells  and  the 
formation  of  the  male  gametes  or  spermatozoa — spermatogenesis.  This  process  in- 
variably consists  of  two  cell  divisions,  one  immediately  following  the  other.  The  first 
is  invariably,  by  a  peculiar  modification  of  the  mitotic  changes,  known  as  heterotype 
mitosis;  the  second,  by  the  usual  or  homotype  mitosis.  As  it  is  more  easy  to  repre- 
sent the  changes  diagrammatically  where  the  chromosomes  are  few,  a  cell  with  four 
chromosomes  is  chosen,  and  as  it  is  more  easy  to  describe  the  process  where  the 
number  of  resulting  spermatozoa  is  small,  as  in  mammals,  where  there  are  always 
four,  than  among  others  where  there  may  be  great  numbers  of  spermatozoa,  it  will 
be  assumed  that  it  is  the  germinal  cell  of  a  mammal  that  is  under  consideration. 


When  the  time  of  functional  activity  arrives  these  cells 
which   in    the    higher    animals    are   known    as  primary 


206  BIOLOGY:  GENERAL  AND  MEDICAL 

spermatocytes,  manifest  certain  peculiar  proliferative 
activities,  the  chief  of  which  is  known  as  the  reduction 
division.  The  chromatic  substance  in  the  nucleus  gathers 
together  to  form  the  usual  number  of  chromosomes,  but  in- 
stead of  assuming  their  customary  appearance  and  arrange- 
ment they  appear  -in  pairs  or  gemmini.  This  conjugation 
of  the  chromosomes  leads  to  an  appearance  easily  misin- 
terpreted to  mean  that  the  cell  has  either  twice  the  usual 
number,  or  only  half  of  the  usual  number.  The  appearance 
of  the  nuclear  spindle  is  now  followed  by  the  separation  of 
the  gemmini,  and  the  passage  of  each  component  chromo- 
some to  an  opposite  pole,  so  that  the  resulting  two  cells — 
secondary  spermatocytes — receive  whole  chromosomes,  and 
only  half  of  the  usual  number.  This  is  the  reduction  di- 
vision, and  is  described  as  heterotype  mitosis.  After  a 
short  interval  each  secondary  spermatocyte  again  divides 
into  a  number  of  small  cells — spermatozoa — varying 
among  different  animals.  This  division  is  effected  by 
the  homotype  mitosis,  each  resulting  cell  receiving  parts 
of  chromosomes,  but  always  the  reduced  number. 

In  man  and  probably  most,  if  not  all,  mammals  the 
primary  spermatocyte,  with  the  full  somatic  number  of 
chromosomes,  gives  rise,  by  the  heterotype  mitosis,  to  two 
secondary  spermatocytes  with  the  reduced  number,  and 
each  of  these,  by  homotype  mitosis,  to  two  spermatozoa, 
each  with  the  reduced  number.  The  four  spermatozoa 
(gametes)  are  of  uniform  size,  similar  in  appearance,  and, 
so  far  as  is  known,  of  equal  functional  value. 

Ob'genesis. — The  germinal  cells  of  the  female  are  sim- 
ilarly set  aside  in  the  gonads — ovaries — and,  like  those  of 
the  male,  undergo  multiplication  during  the  period  of 
growth  and  development  by  the  usual  karyokinetic 
changes — homotype  mitosis.  After  the  perfected  develop- 
ment of  the  higher  organisms — mammals — they  seem  to 
undergo  no  further  increase,  but  remain  inactive  until 
sexual  perfection  and  its  various  activities  arouse  them 
to  certain  changes  described  as  maturation.  The  cells 
mature  one  by  one,  and  prepare  for  fertilization  by  a  series 
of  mitotic  changes  analogous  to  those  of  the  opposite  sex. 


REPRODUCTION 


207 


The  beginning  of  the  maturation  is  shown  by  prepara- 
tion for  the  reduction  division.  The  chromatic  substance 
gathers  into  the  usual  number  of  chromosomes,  which 
form  gemmini  and  take  their  usual  position  in  the  nuclear 


Germinal  cell — oocyte  (ovule) — at  rest. 


Oocyte  in  process  of  maturation,  showing  the  four 
chromosomes,  two  (P  and  p)  of  paternal,  and  two 
(M  and  m)  of  maternal  origin. 


Conjugation  of  the  chromosomes,  the  appearance  cor- 
responding with  what  is  seen  in  spermatogenesia. 


Separation  of  the  conjoined  chromosomes  and  reduc- 
tion division — heterotype  mitosis — through  the 
movement  of  entire  chromosomes  to  each  polar  field. 


Result  of  the  reduction  division,  one  large  cell  and  one 
tiny  cell  (polar  body),  each  with  two  chromosomes. 


Homotype    mitosis.     Divisions    of    each    chromosome 
into  two,  and  passage  of  each  into  a  new  cell. 


End-result,  four  cells  each  with  the  reduced  number  of 
chromosomes;  one  cell,  the  ovum  or  gamete,  being 
large  and  functional ;  the  other  three — polar  bodies — 
being  functionless  and  abortive. 

FIG.  82. — Oogenesis. — Diagram  showing  the  changes  attending  the  maturation  of 
the  ovum  and  the  reduction  division  incidental  to  the  formation  of  the  female  gamete. 
As  in  spermatogenesis,  oogenesis  is  accompanied  by  two  cell  divisions,  one  occurring 
immediately  after  the  other,  the  first  or  reduction  division  being  by  heterotype, 
the  second,  by  homotype  mitosis.  In  all  higher  animals  and  plants,  the  effect  is  to 
diminish  the  number  of  chromosomes  in  the  gamete  to  one-half  of  the  somatic 
number.  Unlike  spermatogenesis,  the  four  cells  resulting  from  the  divisions  are 
not  of  equal  valence.  One,  the  egg,  is  functional  and  is  of  large  size,  three  are  minute 
and  of  no  known  value.  Again  a  mammalian  cell  with  four  chromosomes  is  used 
for  convenience  in  the  diagram. 


spindle.  The  gemmini  then  separate,  each  gemminus 
turning  to  the  pole  of  the  cell  opposite  to  its  fellow.  The 
division,  therefore,  ends  in  the  appearance  of  two  cells, 
each  with  one-half  the  somatic  number  of  chromosomes. 


208  BIOLOGY:  GENERAL  AND  MEDICAL 

These  cells,  unlike  the  secondary  spermatocytes,  are  not 
of  uniform  size.  One  is  very  large,  the  other  very  small 
The  reduction  division  is  immediately  followed  by  division 
of  both  of  the  newly  formed  cells,  but  this  time  by  homo- 
type  mitosis,  so  that  all  of  the  four  resulting  cells  contain 
the  reduced  number  of  chromosomes.  Again  the  division 
lacks  uniformity,  the  large  cell  again  dividing  into  a  large 
and  a  small  cell,  and  the  small  cell  into  two  small  cells  of 
uniform  size.  The  final  outcome  is  four  cells,  of  which 
one,  the  ovum,  is  large,  and  three,  the  so-called  polar 
bodies,  very  small  in  comparison.  The  ovum  seems  to  be 
the  only  functionally  active  cell  (gamete),  the  others  are 
abortive  and  disappear  from  view  without  subserving 
any  known  function. 

The  purpose  of  reducing  the  chromosomes  in  this 
manner  seems  to  be  two-fold:  first,  to  prevent  the  cells 
from  becoming  burdened  with  an  overwhelming  number 
of  chromosomes,  as  must  occur  if  they  were  doubled  with 
each  generation  of  organisms,  and  second,  to  permit  the 
admission  to  the  zygocyte,  or  fertilized  egg,  of  an  equal 
quantity  of  essential  substance  (chromosomes)  from  each 
parent,  amphimixis,  a  matter  the  importance  of  which 
will  be  better  understood  when  the  subject  of  conformity 
to  type  has  been  discussed. 

The  nucleus  of  the  spermatozoon  with  its  reduced 
number  of  chromosomes  is  known  as  the  male  pronucleus; 
that  of  the  ovum  with  its  reduced  number  of  chromo- 
somes as  the  female  pronucleus. 

Fertilization  is  effected  by  the  entrance  of  the  male 
pronucleus  into  the  female  cell  whose  nucleus  appears  to 
advance  to  meet  it.  Coming  together  near  one  pole 
of  the  cell,  the  two  pronuclei  conjugate,  mingle  their 
chromosomes,  and  so  form  a  new  nucleus  for  the  zygote 
or  fertilized  cell  which  thus  comes  into  possession  of  the 
full  somatic  number  of  chromosomes. 

The  process  of  chromosome  reduction  generally  per- 
vades the  world  of  multicellular  living  beings.  Wherever 
definite  gametes  are  produced,  reduction  of  chromosomes 


REPRODUCTION  209 

occurs.     Related  phenomena  also  make  their  appearance 
among  unicellular  organisms. 

Though  insufficient  data  are  at  hand  to  enable  accurate 
generalization  to  be  made,  it  seems  safe  to  assume  that 
in  such  plants  and  animals  as  are  subject  to  parthenoge- 
netic  development,  or  development' from  unfertilized  eggs 
or  germinal  cells,  reduction  of  chromosomes  does  not  occur. 

Many  interesting  examples  of  the  special  means  by 
which  reduction  of  chromosomes  is  effected  among 
plants  might  be  given,  but  at  an  expense  of  space  that 
would  scarcely  be  worth  while  in  a  writing  not  particu- 
larly devoted  to  plant  physiology.  In  dismissing  the 
subject,  however,  one  fact  should  be  mentioned,  that  is, 
that  among  the  ferns,  mosses,  and  algae,  where  alterna- 
tion of  generations  exists,  the  asexual  generations 
(sporophytes)  are  diploid — i.e.,  have  spores  possessing 
the  somatic  number  of  chromosomes,  while  the  sexual 
generations  (gametophytes)  are  haploid — i.e.,  produce 
sexual  cells,  or  gametes,  having  the  reduced  number; 
and  also  that  among  animals,  such  as  certain  nematode 
worms — angiostomum — in  which  hermaphroditic  genera- 
tions alternate  with  bisexual  generations,  the  hermaphro- 
ditic individuals  have  the  diploid,  and  the  bisexual  gen- 
erations the  haploid  number  of  chromosomes. 

The  fertilized  cell,  or  zygote,  is  immediately  ready  for 
development  into  the  new  individual,  which,  through 
the  receipt  of  an  equal  number  of  chromosomes  from 
each  parent,  inherits  characteristics  from  each.  The 
development  of  the  zygote  into  the  new  individual  forms 
a  new  phase  for  study,  known  as  ontogenesis. 

Before  proceeding,  however,  it  may  be  well  to  inquire 
whether,  in  the  present  state  of  knowledge,  we  are  jus- 
tified in  attributing  to  the  chromosomes  of  the  male 
and  female  pronuclei  the  source  of  parental  and  ma- 
ternal characters.  This  subject  will  be  considered  at 
some  length  in  a  future  chapter,  but  it  seems  wise  at 
present  to  say  a  word  or  two  concerning  the  evidence. 
Boveri  has  succeeded  in  rearing  an  echinoderm  larva, 
14 


210  BIOLOGY:  GENERAL  AND  MEDICAL 

exhibiting  only  paternal  characters  from  the  enucleated 
egg  of  one  species  fertilized  by  the  sperm  of  another. 
This  seems  to  be  conclusive,  but  it  is  apparently  offset 
by  an  experiment  of  Kupelweiser  and  Loeb,  who  obtained 
a  larva  showing  no  paternal  characters  at  all  from  the 
ovum  of  a  sea-urchin,  fertilized  by  the  spermatozoon  of 
a  mollusc.  However,  in  the  latter  case  we  can  be  fairly 
sure  that  the  egg  was  induced  to  develop  partheno- 
genetically  through  contact  with  stimulating  substances 
contained  in  the  molluscan  spermatozoon,  and  that  no 
amphimixis  took  place,  as  the  heterologous  sperm  cells 
always  die  in  an  ovum  of  such  distant  relationship,  and 
the  paternal  chromosomes  are  lost  in  consequence. 

In  order  that  ova  shall  develop  they  must  ordinarily 
be  fertilized,  but  this  is  not  in  all  cases  essential.  J. 
Loeb  has  done  much  to  convince  us  that  the  stimuli  that 
lead  to  development  are  chemical  or  mechanical,  and  has 
experimentally  treated  a  variety  of  unfertilized  eggs  in 
such  manner  as  to  bring  about  artificial  parthenogene- 
sis. In  natural  sexual  fertilization  the  spermatozoon 
accomplishes  the  double  purpose  of  furnishing  the  neces- 
sary stimulus  at  the  same  time  that  it  effects  amphimixis, 
and  thus  affords  opportunity  for  the  variation  of  the 
species. 

REFERENCES. 
Same  as  for  Chapter  VII. 


CHAPTER  IX. 
ONTOGENESIS. 

Every  living  thing  begins  its  existence  as  a  single  cell, 
a  condition  of  primitive  simplicity,  and  finally  arrives  at 
a  varying  degree  of  complexity,  according  to  its  phy- 
logeny.  The  study  of  the  intervening  transformations 
through  which  each  organism  must  pass  is  known  as 
ontogenesis  or  ontogeny.  During  the  early  stages  of  de- 
velopment, there  is  no  resemblance  between  the  parent 
organism  and  the  growing  germ  which  is  known  as  an 
embryo.  The  study  of  embryos  and  their  development 
is  called  embryology. 

When  the  embryo  of  one  of  the  higher  animals  reaches 
a  certain  point,  and  has  developed  sufficiently  to  enable 
its  specific  characters  to  be  recognized,  it  becomes 
known  as  a,  foetus. 

If,  as  in  certain  cases,  the  embryo  becomes  self-sustain- 
ing and  independent,  without  attaining  parental  re- 
semblance and  continues  for  some  time  in  this  "semi- 
developed"  form,  it  is  described  as  a  larva.  In  a  few 
forms  of  life,  the  larvse  of  the  tapeworms  Coenurus  and 
Echinococcus,  and  in  certain  dipterous  midges,  a  peculiar 
form  of  parthenogenetic  reproduction  takes  place  in  the 
larvae.  To  it  the  term  pcedogenesis  has  been  applied. 

The  early  writers  upon  the  science  of  development, 
having  no  data  upon  which  to  build,  were  obliged  to 
content  themselves  with  theoretical  speculations,  most 
of  which  are  of  little  interest  to-day,  yet  they  deserve 
consideration  and  are  useful  as  exemplifying  how  wide 
of  the  truth  theory  may  lead  and  how  difficult  it  may 
be  for  it  to  give  place  to  fact. 

Until  the  time  of  William  Harvey  (1578-1657)  the 
whole  subject  of  "generation"  was  so  obscure  as  to 
211 


212  BIOLOGY:  GENERAL  AND  MEDICAL 

merit  scant  attention.  He  took  up  the  subject  from  the 
experimental  side  and  brought  it  to  the  point  from  which 
no  departure  has  since  been  possible,  namely,  that  "the 
egg  is  the  common  beginning  of  all  animals"  (Ovum  esse 
primordium  commune  omnibus  animalibus).  The  dis- 
covery of  the  spermatozoon  was  made  in  1677  by  Ham- 
men.  He  showed  these  little  bodies  to  Leeuwenhoek, 
who  studied  them  with  enthusiasm  and  diverted  further 
attention  from  being  bestowed  upon  the  egg  by  declaring 
the  spermatozoa  to  be  the  essential  germs  and  that  in 
them  were  present  the  beginnings  of  the  future  soul.  It 
was  even  believed  that  they  were  minute  living  animals 
of  both  sexes,  capable  of  coition,  etc.,  and  the  philosopher 
Leibnitz  declared  them  immortal.  Scientists  and  philos- 
ophers soon  became  divided  into  two  schools,  the  Ovists 
and  the  Ammalculists.  As  the  ideas  of  the  Animal- 
culists  departed  so  far  from  the  truth  as  to  find  no  place  in 
modern  thought,  they  can  be  dismissed  with  the  remark 
that,  following  Plantade  (1699),  they  eventually  came  to 
see  in  the  human  spermatozoon  a  complete  miniature 
of  the  human  fcetus,  enclosed  in  its  membranes,  its 
head  bowed  upon  its  breast,  and  its  limbs  flexed — the 
"homonculus" — and  supposed  that  when  such  an  entity 
was  properly  received  by  the  uterus  it  proceeded  to  grow 
into  a  human  being. 

The  Ovists,  on  the  other  hand,  regarded  the  sperma- 
tozoon with  comparative  indifference.  Some  believed 
it  to  be  a  parasitic  animalcule  of  the  semen,  others  con- 
ceived that  it  carried  some  stimulating  force  by  which 
the  growth  of  the  egg  was  stimulated.  They  all  agreed 
that  it  was  in  the  ovum  that  the  future  being  was  con- 
tained. The  ideas  of  the  philosophically  minded  of  this 
school  eventually  crystallized  into  the  "  preformation 
theory,"  or  "theory  of  evolution,"  by  which  it  was  sup- 
posed that  the  ovum  of  every  animal  contained  a  minia- 
ture of  the  future  adult,  -complete  in  every  detail,  and 
only  requiring  nourishment  in  order  that  it  should 
grow  larger  and  larger  until  the  adult  size  was  reached. 
"There  is  no  such  thing  as  becoming,"  is  the  way  it  is 


ONTOGENESIS  213 

expressed  by  Haller  in  the  "Elements  of  Physiology;" 
"No  part  in  the  animal  was  formed  before  another:  all 
were  created  at  the  same  time."  "Against  such  contra- 
dictory evidence  as  the  metamorphoses  of  insects,  the 
preformationists  had  none  but  verbal  weapons  and 
dogmatic  opinions  that  found  expression  in  the  state- 
ment that  though  it  might  not  be  in  a  visible  form,  still 
the  caterpillar  contained  in  itself  the  pupa,  and  the  pupa 
the  butterfly,  therefore  the  butterfly  was  already  present, 
as  such,  in  the  caterpillar." 

Successive  generations  were  accounted  for  by  sup- 
posing that  the  human  ovary  not  only  contained  num- 
bers of  ova,  each  containing  an  individual  in  miniature, 
but  that  in  the  ovary  of  this  minature  there  were  many 
other  and  smaller  miniatures,  and  within  these  still 
others,  and  so  on,  like  the  Japanese  nests  of  boxes,  one 
within  another.  "In  the  extension  of  this  box-wiihin- 
box  doctrine  (Einschachtelungslehre)  the  distinguished 
physiologist,  Haller,  calculated  that  God  had  created 
together,  6000  years  ago — on  the  sixth  day  of  his  crea- 
torial  labors— the  germs  of  200,000,000,000  men,  and 
ingeniously  packed  them  all  in  the  ovary  of  our  venerable 
mother  Eve."  This  was,  of  course,  all  theory,  but  there 
seemed  to  be  no  disposition  to  get  at  the  true  facts.  The 
theory  of  epigenesis,  or  development  of  the  embryo, 
taught  by  Hippocrates  and  Aristotle,  was  almost  for- 
gotten, until  Caspar  Frederich  Wolff  (1735-1794)  again 
brought  it  into  prominence  by  studies  of  the  developing 
hen's  egg,  in  which  he  found  no  preformed  individual, 
but  one  growing,  transforming,  and  differentiating  in  a 
manner  easy  of  study  and  demonstration.  In  his  doc- 
tor's dissertation  (1759)  Wolff  laid  down  the  scientific 
principle  that  what  one  could  not  recognize  by  means 
of  the  senses  was  certainly  not  present  preformed  in  the 
germ.  "At  the  beginning,"  so  he  maintained,  "the 
germ  is  nothing  else  than  an  unorganized  material  elimi- 
nated from  the  sexual  organs  of  the  parent,  which  grad- 
ually becomes  organized,  but  only  during  the  process 
of  development,  in  consequence  of  fertilization." 


214  BIOLOGY:  GENERAL  AND  MEDICAL 

So  strong,  however,  were  the  preformationists  that 
Wolff  failed  to  make  much  impression,  and,  although  a 
simple  investigation  might  have  satisfied  any  scientific 
observer  of  error,  the  real  revival  of  epigenesis  was  deferred 
for  nearly  a  century. 

"In  1672  DeGraaf  described  the  structure  of  the  ovary 
in  birds  and  mammals,  observed  the  ovum  in  the  oviduct 
of  the  rabbit,  and  repeated  Harvey's  statement  as  to  the 
universal  occurrence  of  ova,  although  he  mistook  for  the 
ova  the  follicles  that  now  bear  his  name" — Graafian 
follicles.  In  1827  Carl  Ernst  von  Baer  definitely  traced 
the  mammalian  ovum  from  the  uterus,  through  the  oviduct 
to  its  origin  in  the  Graafian  follicle  in  the  ovary. 

It  is  difficult  to  trace  the  early  discoveries  appertaining 
to  fertilization.  The  fact  that  it  is  necessary  for  the 
ovum  to  contact  with  the  seminal  fluid,  in  order  that 
fecundation  may  take  place,  seems  to  date  from  the 
time  of  Swammerdan  (died  1685);  that  the  spermatozoa 
were  inseparably  connected  with  it  from  the  time  of 
Hartsoeker  (1665-1725).  In  1780  Spallanzani  artificially 
fertilized  the  egg  of  the  frog  and  the  tortoise,  and  even 
successfully  introduced  seminal  fluid  into  the  uterus  of  a 
bitch,  but  made  the  mistake  of  believing  that  it  was  the 
fluid  of  the  semen  that  caused  the  fertilization.  This 
error  was  pointed  out  in  1824  by  Prevost  and  Dumas, 
who  found  that  filtration  destroyed  the  fertilizing  power 
of  the  spermatic  fluid.  The  observation  that  the  sperma- 
tozoon of  the  rabbit  actually  entered  the  ovum  was  first 
made  by  Barry  in  1843.  The  first  to  trace  the  develop- 
ment of  all  the  tissues  from  the  primordial  germinal  cells 
to  the  stage  of  complete  evolution  seems  to  have  been 
Theodore  Schwann  (1839). 

The  starting  point  of  embryological  development,  as 
seen  in  the  light  of  present  scientific  knowledge,  has 
been  reached  in  the  chapter  upon  Reproduction  where 
the  germinal  cells,  early  set  aside  in  the  gonads,  or  repro- 
ductive organs,  maturing  as  gametes,  pass  through  the 
period  of  maturation  characterized  by  the  chromosome 
reduction.  Following  this  comes  the  conjugation  proc- 


ONTOGENESIS  215 

ess  known  as  fertilization,  by  which  the  zygote  or  ferti- 
lized cell  receives  an  equal  quantity  of  essential  nuclear 
(chromosome)  substance  from  each  gamete,  and  therefore 
from  each  parent  of  the  individual  about  to  be  formed. 

It  has  already  been  pointed  out  that  the  ovum,  not 
being  motile,  is  in  a  certain  sense  passive;  the  sperma- 
tozoon, which  is  motile,  active  in  the  process.  The 
spermatozoon  is  no  doubt  attracted  to  the  ovum  by  that 
elementary  characteristic  of  protoplasm  already  de- 
scribed as  chemotropism. 

The  fertilization  is  differently  effected  according  to 
the  differing  conditions  of  life.  Thus,  when  the  animals 
are  aquatic,  the  ova  and  spermatozoa  may  be  discharged 
into  the  surrounding  water  and  their  conjugation  trusted 
to  chance.  When  they  are  terrestrial,  special  means 
must  be  taken  to  overcome  the  obstacles  of  gravity, 
etc.,  and  means  provided  for  conveying  the  spermatozoa 
to  the  ova.  Many  means  used  by  plants  for  effecting 
fertilization  have  already  been  discussed.  Terrestrial 
animals  are  usually  furnished  with  sexual  organs  fitted 
for  coitus  so  that  the  sperm  of  the  male  may  be  directly 
introduced  into  the  organs  of  the  female  where  fertiliza- 
tion takes  place. 

In  plants  the  external,  in  animals  the  internal,  morpho- 
logical characters  predominate  in  importance.  This 
occasions  certain  fundamental  differences  in  embryology 
by  which  the  development  of  the  plants  becomes  a  sepa- 
rate subject,  to  describe  which  would,  on  account  of  the 
diversified  forms  to  be  considered,  divert  us  from  the 
general  scope  of  this  writing.  It  must,  therefore,  be 
left  to  those  intending  to  pursue  botany  as  a  specialty, 
and  be  dismissed  with  the  brief  statement  that  it  con- 
forms to  the  general  principle  that  embryological  develop- 
ment is  the  passage  of  the  organism  from  the  simplicity 
of  unicellular  structure  to  the  complexity  of  differen- 
tiated multicellular  structure,  and  that  in  this  trans- 
formation the  embryo  passes  through  a  series  of  stages 
which  suggest  the  phylogenetic  ascent  of  its  kind.  The 


216  BIOLOGY:  GENERAL  AND  MEDICAL 

significance  of  this  expression  will  be  better  understood 
after  the  perusal  of  the  matter  that  is  to  follow. 

Every  metazoan  begins  its  life  history  as  a  single  cell 
or  egg,  and  whether  this  is  a  distinctly  differentiated 
germinal  cell  or  egg  or  an  indistinctly  differentiated 
germinal  cell  such  as  forms  the  starting  point  of  the 
gemmation  of  ccelenterates,  etc.,  makes  no  essential 
difference. 

For  the  present,  however,  we  shall  neglect  the  undif- 
ferentiated  and  consider  only  the  differentiated  germinal 
cells— the  true  eggs. 

These  present  a  great  variety  of  appearances,  but 
little  difference  of  structure,  as  each  is  a  single  cell.  They 
vary  from  a  size  so  small  as  to  be  microscopic  to  several 
pounds  in  weight  (ostrich  egg),  may  be  naked  and  purely 
protoplasmic  or  covered  with  membranous,  leathery,  or 
calcareous  encasements,  these  differences  serving  to 
enable  even  a  beginner  to  realize  that  there  are  phylo- 
genetic  differences  even  among  the  eggs  themselves. 
No  doubt,  increasing  familiarity  with  eggs  in  general 
will  eventually  show  that  the  differences  are  not  only 
such  as  to  enable  eggs  to  be  referred  to  their  respective 
phyla,  but  to  their  respective  classes,  orders,  genera, 
and  even  species,  as  can  readily  be  done  at  present,  for 
example,  with  birds'  eggs  and  many  insects'  eggs. 

Not  only  are  there  such  external  differences,  but  there 
are  also  striking  internal  differences  among  eggs,  which 
not  only  assist  in  their  classification,  but  also  assist  in 
explaining  peculiarities  attending  their  development. 

Thus,  a  superficial  examination  enables  one  to  separate 
eggs  into  those  that  are  holoblastic,  or  without  yolks, 
and  those  that  are  meroblastic  and  have  yolks,  and  to 
discover  that  though  the  eggs  differ  in  size,  as  do  the 
other  cells  of  the  respective  animals  to  which  they  belong, 
the  presence  or  absence  of  a  yolk  and  the  size  of  that 
yolk  have  much  to  do  with  the  size  of  the  egg.  The 
yolk  is,  moreover,  inclosed  in  the  egg,  which,  according 
to  its  size,  is  surrounded  by  a  thicker  or  thinner  proto- 
plasmic envelope.  The  yolk  which  is  composed  of 


ONTOGENESIS  217 

deuteroplasm  is  intended  to  nourish  the  developing 
embryo,  hence  the  magnitude  of  the  yolk  must  bear 
some  reference  to  the  dependence  of  the  embryo  upon 
that  form  of  nourishment.  In  cases  in  which  the  egg  is 
quickly  developed  into  a  self-sustaining  larva,  there  is 
no  yolk;  in  cases  where  it  becomes  attached  to  the 
uterine  wall  of  the  parent  from  whom  it  derives  nourish- 
ment, the  yolk  may  be  inconspicuous,  but  in  those  cases 
— the  birds,  reptiles,  fishes — where  the  egg  is  entirely 
separated  from  the  parent  and  completes  its  embryonal 
transformations  without  a  larval  form  in  which  addi- 
tional nourishment  can  be  secured  from  without,  the 
yolk  must  be  large  enough  to  supply  all  of  the  embryonal 
requirements. 

The  encumbrance  of  the  yolk  modifies  the  earliest 
transformations  of  the  egg — cleavage — as  will  be  shown. 

Before  leaving  the  eggs  it  is  necessary  to  give  brief 
attention  to  the  subject  of  fertilization.  When  an  egg 
is  surrounded  by  a  leathery  or  calcareous  covering  before 
expulsion  from  the  maternal  body,  it  cannot  be  subse- 
quently fertilized,  so  that  in  such  cases  the  spermatozoa 
must  have  been  emitted  into  the  maternal  organs,  where 
they  meet  and  fertilize  the  egg  before  its  final  coverings 
are  provided,  unless  such  coverings  contain  one  or  more 
openings — micropylae — for  the  special  purpose  of  admit- 
ting the  spermatozoa. 

The  developmental  process  begins  by  cell  division, 
"cleavage,'1  or  "segmentation"  of  the  ovum,  which  is 
followed  by  that  cellular  multiplication  through  the 
continuance  of  which  the  different  tissues  and  organs 
are  produced. 

As  a  rule,  each  egg  gives  rise  to  a  single  individual. 
There  are,  however,  rare  exceptions,  in  which  several 
embryos  may  develop  from  a  single  ovum.  This  phe- 
nomenon, known  as  polyembryony,  was  first  observed  in 
plants,  but  occasionally  appears  in  animals.  Thus,  En- 
cyrtus  is  an  insect  whose  habit  is  to  deposit  its  eggs  in 
the  eggs  of  a  moth,  Hyponomenta.  From  each  egg  a 
morula  embryo  develops,  which  instead  of  proceeding  to 


218  BIOLOGY:  GENERAL  AND  MEDICAL 

develop  according  to  the  plan  later  to  be  described,  di- 
vides so  as  to  form  a  long  chain  of  morulae,  from  each  of 
which  a  separate  embryo  develops. 

Polyembryony  also  appears  occasionally  among  animals 
as  high  as  mammals.  The  researches  of  Fernandez  and  of 
Neuman  and  Patterson  have  shown  that  each  fertilized 
ovum  of  the  armadillo  produces  four  embryos.  Here  the 
polyembryony  is  regular  and  constant.  In  man  the  occa- 
sional occurrence  of  monochorionic  twins  (homologous 
twins)  from  the  development  of  a  single  ovum  may  be  con- 
sidered an  "accidental"  occurrence  of  polyembryony. 

The  mode  of  segmentation  differs  in  different  eggs, 
partly  through  peculiarities  of  the  eggs,  partly  through 
inherited  impulses.  Hertwig  presents  the  following 
scheme  of  cleavage: 

I.  Type. — Holoblastic  eggs  (without  yolks). 

Total  cleavage:  a.  Equal  cleavage  (lower  inverte- 
brates and  mammals),  b.  Unequal  cleavage  (mol- 
lusk  and  amphibia). 

II.  Type. — Meroblastic  eggs  (with  yolks). 

Partial  cleavage:  a.  Discoidal  cleavage  (fishes, 
birds,  and  reptiles),  b.  Superficial  cleavage  (in- 
sects and  arthropods). 

Thus  it  is  at  once  apparent  that  the  presence  or  ab- 
sence of  a  considerable  yolk  determines  whether  the 
cleavage  shall  be  total  or  partial,  and  the  examination 
of  any  thorough  description  of  the  development  of  the 
chick  will  make  clear  the  manner  in  which  the  enormous 
yolk  modifies  segmentation. 

So  soon  as  segmentation  has  begun  it  is  possible  to 
recognize  a  chief  or  primary  axis  and  to  differentiate 
an  animal  pole  and  a  vegetative  pole.  Those  cells  that 
arise  in  the  neighborhood  of  the  animal  pole  give  rise 
to  the  ectoderm  from  which  the  integument,  the  nervous 
system,  the  glands;  the  organs  of  special  sense,  etc., 
develop,  which,  so  to  speak,  preside  over  the  animal 
function.  Those  from  the  opposite  pole  comprise  the 
cells  of  the  entoderm,  from  which  arise  the  digestive  and 


ONTOGENESIS 


219 


FIG.  83. — Cleavage  and  gastrulation  (not  drawn  to  scale).     The  vertical  rows 

A,  B,  C,  and  D  represent  different  classes  of  ova.     A,  an  ovum  with  little  yolk; 

B,  one  with  considerable  yolk  collected  at  the  lower  pole  (p.p) ;  C,  one  with  a 
large  amount  of  dense  yolk  crowding  the  protoplasm    to  one  side    (a.p) ;  D, 
ovum  with  dense  yolk  collected  at  center.     The  numerals  (1-4)  indicate  stages 
in  cleavage  and  gastrulation:  1,  ova;  2,  4-8  celled  stages  of  segmentation;  3, 
blastospheres,  blastula  stage;  4,  gastrula  stage,     a,  Archenteron;  a.p,  active 
pole;  bl,  blastoderm;  bp,  ec,  ectoderm;  en,  entodenn;  ma,  macrospheres;  mi,  mi- 
crospheres;  p.p,  passive  pole;  s.c,  segmentation  cavity;  y,  yolk;  y.c,  yolk  cells. 
(Galloway.) 


220  BIOLOGY:  GENERAL  AND  MEDICAL 

reproductive  organs  which  preside  over  the  vegetative 
functions. 

This  chief  axis  may  be  recognized,  in  large  mero- 
blastic  eggs,  even  before  development  begins,  and  in 
many  cases  is  distinct  as  soon  as  it  begins;  thus,  in  the 
hen's  egg,  the  germinal  vesicle  represents  the  animal 
pole,  the  great  opposed  mass  of  the  yolk  the  opposite  or 
vegetative  pole. 

All  of  the  metazoa  early  show  a  monaxial,  hetero- 
polar  condition  about  which  the  developmental  process 
centres.  In  eggs  with  yolks  the  animal  cells  are  lighter 
than  the  vegetative  cells,  so  that  the  animal  pole  always 
turns  up,  no  matter  in  what  position  the  egg  is  placed. 
In  eggs  without  yolks  and  with  equal  or  fairly  equal 
cleavage  the  animal  cells  distribute  over  the  surface  and 
the  vegetative  cells  arise  within,  so  that  the  vegetative 
pole  is  central. 

The  process  of  cleavage  takes  place  through  karyo- 
kinesis,  the  plane  of  division  being  perpendicular  to  the 
long  axis  of  the  spindle.  Two  cells  are  thus  produced, 
each  of  which  appear  to  possess  an  equal  amount  of 
reproductive  energy  and  an  equality  of  all  the  factors 
concerned  in  development,  for  it  has  been  found  by 
experiment  that  if  these  halves  can  be  separated  and 
the  developmental  process  continued,  as  is  possible 
with  some  of  the  lower  animals,  each  is  able  to  pro- 
gress without  apparent  serious  disturbance  to  complete 
development. 

Further  cleavage  results  through  further  karyokinesis, 
the  poles  of  the  nuclear  spindle  always  being  directed 
toward  the  greatest  protoplasmic  masses,  so  that  there 
result  four,  eight,  sixteen,  thirty-two,  sixty-four,  128 
cells,  or  blastomeres,  and  so  on.  This  process  of  cleavage, 
though  taking  place  after  karyokinetic  changes,  differs 
from  ordinary  cell  division  in  that  it  progresses  so  rapidly 
that  no  time  is  allowed  for  growth  and  the  cells  become 
smaller  and  smaller  as  they  divide. 

In  equal  cleavage  the  cells  of  the  two  poles  are  of 
uniform  size;  in  unequal  cleavage  their  number  is  the 


ONTOGENESIS  221 

same,  but  the  size  varies,  the  animal  cells  being  smaller 
than  the  vegetative  cells.  Equality  of  numbers  is  not, 
however,  preserved,  for  either  the  richness  of  protoplasm 
in  the  animal  half  or  the  magnitude  of  the  cells  of  the 
vegetative  half  determines  that  the  former  outgrows 

SECTION  OF  MORULA. 

SECTION  OF  BLASTULA. 


Archiccele. 


a  b 

FIG.  84. — a,  Section  of  morula;  6,  section  of  blastula.     (Masterman.) 

the  latter  until  with  128  cells  in  the  animal  half  there 
may  be  but  thirty-two  in  the  vegetative  half,  and  so  on. 

The  inequality  of  the  cleavage  is  in  direct  proportion 
to  the  quantity  of  yolk  at  the  vegetative  pole.  Thus, 
in  holoblastic  eggs,  the  cleavage  may  be  equal;  in  the 
enormous  meroblastic  eggs  of  birds  the  yolkless  proto- 
plasm assembled  at  the  animal  pole  is  alone  able  to  un- 
dergo segmentation,  and  the  primitive  assemblage  of  cells 
resulting  from  this  localized  cleavage  appears  as  a  cellu- 
lar disc  floating  upon  the  surface  of  the  yolk.  This 
germinal  disc  receives  the  name  "blastoderm." 

The  segmentation  first  results  in  the  formation  of  a 
solid  cellular  mass  which  bears  a  partial  resemblance 
to  a  mulberry  and  is  known  as  a  morula.  Every  egg 
passes  through  this  stage.  Soon,  however,  the  morula 
becomes  changed  by  assumption  of  fluid  or  by  vacuo- 
lization  of  the  inner  cells,  and  a  hollow  sphere  is  formed, 
the  blastula,  surrounded  on  all  sides  by  a  single  layer  of 
blastomeres. 

The  eggs  of  the  invertebrates  always  form  blastulae, 
some  of  which  are  ciliated,  free-swimming,  and  self- 
sustaining  larval  forms.  Such  are  called  monoblastic 
larva.  They  are  typically  centro-symmetric,  the  hollow 
centre  being  known  as  the  archicele  or  blastocele,  the 
cellular  layer  as  the  archiblast.  The  sponges  have 
larvae  of  this  form. 


222 


BIOLOGY:  GENERAL  AND  MEDICAL 


Holoblastic  eggs   of   higher   animals,   having   formed 
blastula,  next  undergo  a  peculiar  invagination  through 


SECTION  OF  GASTRULA. 

Archenteron 


SECTION  OF  PLANULA. 


the  ascent  of  the  vegetative  pole  of  the  blastula  layer, 
until  it  comes  into  contact  with  the  cells  of  the  animal 
pole.  The  invaginated  blastula  thus  comes  to  resemble 
a  hollow  ball  one  side  of  which  has  been  pressed  in 
against  the  other.  The  result  of  the  invagination  is 
that  the  embryo  now  consists  of  a  double  layer  of  cells, 
surrounding  a  new  cavity  formed  by  the  invagination, 
while  the  original  segmentation  cavity  has  virtually 
been  extinguished.  The  embryo  is  now  called  a  gas- 
trula,  and  is  diploblastic,  because  it  consists  of  an  outer 
layer  of  cells,  the  ectoderm  or  epiblast,  and  an  inner 
layer,  the  entoderm  or  hypoblast.  The  cavity  formed  by 
the  invagination  is  now  known  as  the  archenteron,  and 
becomes  more  and  more  enclosed  by  increase  in  the 
cellular  layers  until  the  original  bell-shape  gives  place  to- 
a  more  spheroidal  form  with  a  central  opening,  the 
blastopore. 

Embryos  at  this  stage  bear  a  distinct  resemblance  to 
certain  larvae  of  crelenterates,  and  indeed  this  diploblastic 
larva  is  the  general  plan  of  development,  as  well  as  the 
foundation  of  structure  of  the  medusae. 

Holoblastic  eggs  of  still  higher  animals  next  progress 
to  the  formation  of  triploblastic  larvae  through  the  forma- 
tion of  a  third  cellular  layer,  the  mesoderm  or  mesoblast, 
composed  of  slightly  differentiated  cells,  which  arise  from 
two  endodermal  rudiments  symmetrically  arranged  on  op- 


ONTOGENESIS  223 

posite  sides  of  the  central  axis.  In  different  embryos  its 
appearance  and  arrangement  vary,  but  it  is  described  by 
Masterman  thus:  "  It  consists  of  a  more  or  less  complex 
double  layer  of  cells  of  which  the  outer  layer  lines  the 
epiblast  and  the  inner  covers  the  hypoblast.  These  two 
layers  enclose  a  spacious  cavity,  called  the  ccelum,  which 
is  usually  filled  with  a  nutrient  fluid.  The  crelum  is  not 
usually  continuous,  but  it  may  be  divided  in  the  median 


FIG.  86.— Section  through  the  germ  disc  of  a  freshly  laid  fertilized  hen's  egg. 
fh.  Cleavage-cavity;  wd,  white  yolk;  vw,  lower  cell  layer;  dw,  upper  cell  layer  of 
the  blastula.  (After  Duval.) 

plane  by  dorsal  and  ventral  mesenteries,  which  are 
double  and  serve  to  support  the  hypoblastic  canal;  or 
it  may  be  divided  up  by  lateral  mesenteries  or  septa 
running  transversely  to  the  long  axis  of  the  organism. 
The  mesoblastic  walls  later  form  the  muscles,  skeletal 
tissues,  gonads,  and  partly  the  excretory  organs;  and 
the  ccelum  often  communicates  with  the  exterior  by 
paired  canals  called  nephridia." 

An  embryo  arrived  at  this  degree  of  complexity  will  be 
found  to  conform  fairly  well  in  structure  with  that  of 
the  unsegmented  worms,  though  it  may  not  otherwise 
resemble  them. 

If  we  now  digress  to  see  how  the  early  development  of 
meroblastic  differs  from  that  of  holoblastic  eggs,  we  find 
the  dissimilarities  chiefly  accounted  for  by  the  presence 
of  the  yolk.  When  this  is  large,  as  in  the  hen's  egg,  it 
is  impossible  for  blastulation  and  gastrulation  to  take 
place  in  the  manner  described.  Instead,  the  segmenta- 


224  BIOLOGY:  GENERAL  AND  MEDICAL 

tion  is  partial  (discoidal)  and  limited  to  a  superficial 
area  where  it  forms  a  "germinal  disc"  or  group  of  cells 
which,  so  to  speak,  floats  upon  the  upper  surface  of  the 
egg.  This  germinal  disc  after  developing  to  a  certain 
point  undergoes  differentiation  into  a  superficial  layer 
and  deeper  layers  which  are  separated  by  a  narrow 
interval  or  space,  which  constitutes  the  cleavage  cavity. 
Thus,  through  a  modification  necessitated  by  circum- 
stances, the  homologue  of  the  blastula  is  produced.  As 


FIG.  87. — Two  germ  discs  of  hen's  egg  in  the  first  hours  of  incubation,  df,  Area 
opaca;  hf,  area  pellucida;  s,  crescent;  sk,  crescent-knob;  es,  embryonic  shield; 
pr,  primitive  groove.  (After  Roller.) 

the  blastula  is  perfected,  gastrulation  takes  place  not 
by  the  simple  invagination  of  one  side  of  the  sphere, 
for  the  segmentation  cavity  is  not  spherical,  but  through 
a  combination  of  folding  and  invagination  by  which  one 
portion,  the  outer  layer,  forming  an  ectoderm,  comes  to 
overlie  the  inner  layer,  the  entoderm,  and  form  a  kind 
of  blind  sac,  the  slit-like  opening  into  which  is  the 
blastopore. 

If  at  this  time  the  egg  of  the  hen  could  be  observed 
from  the  external  surface  over  the  germinal  disc,  one 
would  see  an  oval  area  undermined  posteriorly.  At  the 
centre  of  the  posterior  flap  a  little  notch  soon  makes  its 
appearance,  which  becomes  deeper  and  ends  in  a  groove 
extending  anteriorly  half  the  length  of  the  disc — the 
primitive  groove — in  the  centre  of  which  is  a  linear 
marking,  the  primitive  streak. 

The  primitive  embryo  thus  comes  to  consist  of  two 
germinal  layers,  ectoderm  and  entoderm,  forming  a 


ONTOGENESIS  225 

concavo-convex  plate,  the  convex  surface  of  which  con- 
sisting of  ectoderm  is  destined  to  form  the  dorsum  and 
external  coverings  of  the  embryo,  the  concave  surface 
the  internal  organs  about  which  the  embryo  is  to  grow. 
As  the  growth  proceeds,  the  circumference  of  the  germinal 


Medullary 

furrow 


Primitive  streak 
and  groove 


FIG.  88. — Surface  view  of  area  pellucida  of  an  eighteen-hour  chick  embryo- 
The  area  opaca  is  omitted;  the  pear-shaped  outline  marks  the  limit  of  the  area 
pellucida.  At  the  place  where  the  two  medullary  folds  are  continuous  with 
each  other  there  is  to  be  seen  a  short  curved  line,  which  represents  the  head 
fold.  In  front  of  it  there  lies  a  second  line  concentric  with  it,  the  beginning 
of  the  amniotic  fold.  (Balfowr.) 

disc  increases,  and  the  subjacent  yolk  is  absorbed  as  it 
furnishes  the  growing  cells  writh  nourishment.  The 
increasing  disc  does  not  grow  uniformly  and  hence  be- 
comes thrown  into  folds  which,  when -viewTed  from  the 
dorsal  surface,  indicate  the  position  of  future  structures. 
Thus  an  anterior  transverse  fold  indicates  where  the  am- 
niotic membrane  is  to  form;  a  second  transverse  fold, 
where  the  head  of  the  embryo  is  to  develop.  The  longi- 
tudinal groove  in  the  anterior  part  of  the  disc  indicates 

15 


226  BIOLOGY:  GENERAL  AND  MEDICAL 

where  the  spinal  cord  will  form,  and  the  folds  on  each 
side  of  the  groove,  two  arches  of  the  ectoderm  by  con- 
crescence or  fusion  of  which  the  spinal  canal  will  event- 
ually he  formed. 

While  these  wrinkles  or  folds  are  preparing  the  way 
for  future  development,  the  mesoderm,  from  which  the 
skeletal,  motor,  circulatory,  and  connective  tissues  are  to 
form,  is  being  prepared  by  the  penetration  into  the  space 
between  the  ectoderm  and  entoderm,  of  a  mass  of  small 
cells  in  many  superimposed  layers,  arising  from  both 
ectoderm  and  entoderm,  and  as  these  cells  become  in- 


ifc  i/c 

Fio.  K9. — Cross-section  through  the  middle  of  the  primitive  streak  of  a  chick's 
germ  disc.  At  some  distance  from  the  primitive  groove  is  to  be  seen  upon  the 
left  side  of  the  figure  in  cross-section  the  marginal  groove  of  HLs;  upon  the  right 
side  it  is  as  yet  little  developed.  ak,  Outer,  ik,  inner,  mk,  middle  germ  layers; 
pr,  primitive  groove;  pa,  primitive  streak;  gr,  marginal  groove.  (After  Roller.) 

closed  between  the  two  chief  blastodermic  layers  they 
also  become  separated  from  the  parent  blastodermic 
layers  and  differentiated  as  a  third  blastodermic  layer. 

A  transverse  section  of  the  germinal  disc  at  the  time 
of  the  formation  of  the  mesoderm,  which  coincides  fairly 
well  with  the  period  of  the  formation  of  the  primitive 
streak  and  groove,  is  shown  in  the  illustration. 

The  future  development  of  such  meroblastic  eggs 
depends  upon  their  character.  If  the  eggs  are  enclosed 
in  shells,  development  is  complicated  by  the  formation 
of  a  membrane  investing  the  embryo — the  amnion;  if 
the  eggs  are  without  shells,  as  those  of  fishes,  there  are 
no  membranes,  and  development  is  less  complicated. 
In  both  cases  the  growth  of  the  embryo  continues  by 
folding,  convergence,  and  concrescence  of  the  cellular 
germinal  plate,  more  and  more  completely  separating 
the  embryo  from  its  yolk. 

Now,  leaving  the  meroblastic  eggs  of  the  fishes,  rep- 


ONTOGENESIS 


227 


tiles,  birds,  and  the  lowest  mammals,  we  return  once 
more  to  holoblastic  eggs  as  we  find  them  in  mammals. 


FIG.  90.  FIG.  91. 

FIG.  90. — Ovum  of   the   bat,  showing  vacuolation  of    the  segmented    egg    to 

form  the  blastodermic  cavity.      X  500.     (Van  Beneden.) 

FIG.  91. — Ovum  of  the  bat,  showing  vacuolation  of  the  segmented  egg  to  form  the 
blastodermic  cavity.     X  600.     (Van  Beneden.) 

These  begin  development  by  an  almost  equal  segmen- 
tation resulting  in  the  formation  of  a  morula,  composed 
of  cells  of  fairly  uniform  size.  When  the  morula  stage 
has  been  completed,  vacuoles  begin  to  appear  in  some 
of  the  central  cells,  become  larger,  coalesce,  and  give 
origin  to  an  irregular  fissure-like  space  which  is  the 


FIG.  92. — Ovum  of  bat;  differentiation  of  embryonic  button.     (Fan  Beneden.) 

segmentation  cavity,  or  lecithocele,  and  brings  the  ovum 
to  the  stage  of  the  blastodermic  vesicle.     Though  not 


228 


BIOLOGY:  GENERAL  AND  MEDICAL 


formed  in  a  manner  identical  with  the  regular  blastula? 
of  the  invertebrates,  the  blastodermic  vesicle  is  easily 
homologized  with  it  and  subserves  the  same  purpose 
as  the  blastula  in  the  subsequent  developmental  stages. 


•afaarni'ofi'e  cari/JF 


^autl-af  azotic 


FIG.  93. — Ovum  of  bat,  showing  amniotic  cavity.     X  200.     (Van  Beneden.) 

The  mammalian  ovum  at  this  stage  consists  of  a 
layer  of  flattened  cells,  the  "  outer  cell  mass,"  surround- 
ing a  cavity,  into  which  an  irregular  mass  of  cells  of 
more  spherical  form,  "the  inner  cell  mass,"  hangs  sus- 
pended from  one  side  encroaching  upon  the  space. 
This  embryo  is  imbedded  in  the  uterine  epithelium 


FIG.   94. — Ovum  of  bat;   differentiation  of  amniotic   cavity.     X  275. 
Beneden.) 

which  apparently  melts  away  below  its  attachment  in 
order  that  placentation  may  be  made  possible  later  on. 

A  free  ovum  in  water  or  an  ovum  inclosed  within  a 
shell  is  able  to  perfect  its  differentiations  undisturbed 
by  contact  with  external  bodies;  but  a  mammalian  ovum 
of  small  size,  without  a  nutrient  yolk  to  feed  upon  and 
situated  in  a  crypt  of  the  uterine  mucosa,  is  obliged  to 
prepare  for  its  future  nutrition  by  effecting  a  communi- 
cation with  the  maternal  supply,  and  arrange  for  its 


ONTOGENESIS 


229 


freedom  from  external  interference  by  surrounding  itself 
with  smooth  membranes  within  which  development  may 
proceed. 

Fish    embryos    are    without    such    membranes;    the 


FIG.  95. — Section  through  embryonic  region  of  ovum.  First  week  of  preg- 
nancy. E.Sch,  Embryonic  epiblast;  Ent,  embryonic  hypoblast;  M es,  embryonic 
mesoblast;  D.S,  umbilical  vesicle;  Ekt,  chorionic  epiblast;  Sp,  fold  in  exocoelom; 
A.H,  amniotic  cavity  lined  by  a  single  layer  of  flattened  cells,  which  are  in  strik- 
ing contrast  with  the  layer  of  cylindric  cells  of  the  embryonic  epiblast.  (H. 
Peters.) 

chick  forms  one  of  them,  the  amnion;  the  mammalian 
ovum  forms  two,  the  amnion  for  protection  and  the 
chorion  for  nutrition.  It  is  not  necessary  to  particu- 
larize in  regard  to  the  amnion  or  other  fostal  membranes 
as  this  chapter  is  not  intended  to  be  an  adequate  descrip- 
tion of  the  developmental  details,  but  simply  to  epitomize 
such  facts  appertaining  to  development  as  shall  show 
the  harmony  existing  between  all  forms  of  life  in  the 
general  plan  upon  which  ontogeny  is  perfected. 

With  the  formation  of  the  amnion,  the  embryo  ad- 
vances to  the  point  where  it  consists  of  a  didermic  plate, 
on  the  uterine  side  of  which  the  primitive  amniotic 
cavity  is  situated,  on  the  other  side  of  which  is  the  yolk 
sac  found. 


230 


BIOLOGY:  GENERAL  AND  MEDICAL 


fis".      &WWfa&. 

FIG.  96. — A  series  of  embryos  at  three  comparable  and  progressive  stages  of 
development  (marked  I,  II,  III),  representing  each  of  the  classes  of  vertebrated 
animals.  (After  Hdckel.) 


ONTOGENESIS 


231 


232  BIOLOGY:  GENERAL  AND  MEDICAL 

This  didermic  plate,  consisting  of  ectoderm  and  ento- 
derm,  between  which  the  cells  of  the  mesoderm  soon 
make  their  appearance,  is  not  essentially  different  from 
the  oval  germinal  disc  floating  upon  the  great  yolk  of  the 
hen's  egg,  and  its  future  development  progresses  in  much 
the  same  way.  In  the  mammalian  egg,  however,  the 
researches  of  Peters  and  van  Beneden  indicate  that 
endoderm  formation  does  not  take  place  through  gas- 
trulation  or  invagination  as  in  the  eggs  of  the  lower 
phyla,  but,  like  the  amnion  formation,  is  arrived  at  by  a 
short  cut — i.e.,  by  vacuolization  of  certain  cells. 

After  the  formation  of  the  embryonic  area  (germ  disc) 
composed  of  its  three  germinal  layers,  the  future  develop- 
mental process  consists  in  a  succession  of  wrinkles  and 
puckers  resulting  from  unequal  growth  of  cells  in  different 
locations,  a  general  tendency  of  the  external  convex 
surface  to  outgrow  and  surround  the  inner  concave 
surface,  the  fusion  or  concrescence  of  many  of  the  folds, 
the  growth  of  groups  of  cells  at  certain  points  to  form 
organs  and  so  bring  about  the  final  evolution  of  the 
fostus. 

The  external  morphological  development  and  the 
resemblances  it  embraces  can  be  more  quickly  understood 
by  an  examination  of  the  accompanying  illustrations 
taken  from  Romanes  than  from  a  lengthy  description. 

Internal  development  progresses  simultaneously  with 
external  development.  Perhaps  so  much  of  this  has 
been  left  to  the  imagination  of  the  reader  that  it  may  be 
well  to  consider  certain  features  more  particularly. 
Let  us  begin  by  remarking  that  it  progresses  simply  or 
complexly  according  to  the  future  simplicity  or  complex- 
ity of  the  species  to  which  the  embryo  belongs.  Let  it 
also  be  understood  that  in  spite  of  complexity  it  pro- 
gresses rapidly,  and  that  the  complexity  incidental  to 
the  phylogenetic  position  of  the  animal  whose  embryo 
is  examined  is  not  the  result  of  the  consecutive  forma- 
tion of  the  internal  organs,  but  of  their  simultaneous 
formation. 


ONTOGENESIS 


233 


Therefore,  in  animals  of  complicated  structure  many 
different  things  are  going  on  at  the  same  time  that  must 
be  separately  and  individually  described.  Again,  as 
the  embryo  of  a  vertebrate  is  at  no  stage  of  its  embryonal 
development  self-supporting,  all  of  the  developmental 
processes  look  toward  future  needs,  making  no  pro- 
vision for  immediate  needs,  which  are  provided  for  by 
the  yolk  or  derived  through  the  placenta  (oxygen,  nu- 
triment, etc.)  from  the  parent. 

Thus,  in  phylogenetic  development,  it  has  been 
shown  that  the  first  need  of  the  organism  is  food,  and 
the  first  specializations  have  to  do  with  the  vegetative 
functions,  so  that  the  vegetative  organs  are  the  first  to 
evolve,  the  first  cell  differentiation  being  the  separation 
of  the  outer  cells  into  protective  coverings  and  the  inner 
cells  into  nutrition  providers.  Following  this  means 


FIG.  97. — The  metamorphosis  of  the  frog.     The  numbers  indicate  the  sequence. 
(Galloway  after  Brehm.) 

must  be  provided  for  transporting  the  nutriment  so 
that  it  may  easily  reach  the  cells  not  contiguous  to  the 
source  of  supply.  The  last  system  to  appear  is  the 
correlating  and  coordinating  central  nervous  system. 


234  BIOLOGY:  GENERAL  AND  MEDICAL 

In  the  ontogenetic  development  of  the  higher  animals 
the  conditions  are  different,  for  the  nourishment  of  the 
embryonal  tissues  is  provided  for  by  the  egg  yolk  or  by 
the  placenta,  so  that  the  digestive  organs  need  not  be 
early  perfected.  The  size  of  the  embryo  and  the  arrange- 
ment of  its  parts  are  also  such  that  the  circulatory 
organs  need  not  be  active  before  a  considerable  general 
complexity  of  structure  is  attained.  There  is,  however, 
among  vertebrates  a  general  dominance  of  the  nervous 
system,  so  that  instead  of  the  organic  systems  developing 
one  after  the  other,  as  in  phylogenetic  progress,  ontoge- 
netic development  is  so  modified  that  the  dominant  sys- 
tem first  makes  its  appearance,  and  in  vertebrate  em- 
bryos the  central  nervous  system  which  takes  precedence 
of  all  others  in  importance  is  one  of  the  first  to  appear. 

However,  though  the  importance  and  hence  the  order 
of  development  is  changed,  the  general  plan  of  develop- 
ment for  each  organ  or  system  of  organs  is  quite  compar- 
able to  that  seen  in  phylogenesis. 

"When  one  traces  the  course  of  development  of  any 
vertebrate,  he  finds,  speaking  in  general  terms,  that 
those  fundamental  characteristics  more  or  less  common 
to  all  vertebrates  first  appear,  being  followed  by  second- 
ary characteristics  distinguishing  one  class  from  another." 
In  vertebrate  embryos,  however,  before  the  develop- 
ment reaches  a  certain  point,  distinct  resemblances  to 
invertebrate  forms  are  met,  and  the  younger  the  embryo 
is,  the  more  it  has  in  common  with  embryos  in  general, 
until  at  the  very  beginning  we  come  to  the  single  germi- 
nal cell  which  is  the  starting  point  of  every  embryo. 
These  facts  have  found  expression  in  the  statement  that 
"the  ontogeny  recapitulates  the  phylogeny."  This  as  a; 
theory  is  certainly  justified;  as  a  fact  it  must  be  further 
explained.  Von  Baer,  in  1828,  gave  us  the  following 
generalizations: 

1.  That  which  is  common  to  a  large  group  of  animals  develops  in 
the  embryo  earlier  than  that  which  is  special. 


ONTOGENESIS  235 

2.  From  the  most  generalized  stage,  structures  less  generalized 
are  developed,  and  so  on  until  the  most  special  appears. 

3.  The   embryo  of  a  given  animal  form,   instead   of  passing 
through  the  other  given  forms,  separates  itself  from  them  more 
and  more. 

4.  Therefore,  the  embryo  of  the  higher  forms  is  never  like  a 
lower  form,  but  only  like  its  embryo. 

These  principles  have  since  been  much  insisted  upon, 
especially  by  Haeckel,  who  termed  the  law  of  recapitula- 
tion the  "  Biogenetic  law." 

The  emphasis  laid  upon  this  "law"  by  many  writers 
has,  however,  given  rise  to  some  mistakes,  as  students 
are  apt  to  think  that  every  embryo  must  and  does  show 
its  complete  phylogeny  in  its  ontogeny. 

In  explaining  this  misconception,  Romanes  says: 

"Supposing  the  theory  of  evolution  to  be  true,  it  must  follow  that 
in  many  cases  it  would  have  been  more  or  less  disadvantageous 
to  a  developing  type  that  it  should  have  been  obliged  to  reproduce 
in  its  individual  representations  all  the  phases  of  development 
previously  undergone  by  its  ancestry — even  within  the  limits 
of  the  same  family.  We  can  easily  understand,  for  example,  that 
the  waste  of  material  required  for  building  up  the  useless  gills  of 
embryonic  salamanders  is  a  waste  which,  sooner  or  later,  is  likely 
to  be  done  away  with;  so  that  the  fact  of  its  occurring  at  all  is 
in  itself  enough  to  show  that  the  changes  from  aquatic  to  terrestrial 
habits  on  the  part  of  this  species  must  have  been  one  of  com- 
paratively recent  occurrence.  Now,  in  as  far  as  it  is  detrimental 
to  a  developing  type  that  it  should  pass  through  any  particular 
ancestral  phases  of  development,  we  may  be  sure  that  natural 
selection — or  whatever  other  adjustive  causes  we  may  suppose 
to  have  been  at  work  in  the  adaptation  of  organisms  to  their 
surroundings — will  constantly  seek  to  get  rid  of  this  necessity, 
with  the  result,  when  successful,  of  dropping  out  the  detrimental 
phases.  Thus  the  foreshortening  of  developmental  history  which 
takes  place  in  the  individual  lifetime  may  be  expected  often  to 
take  place,  not  only  in  the  way  of  condensation,  but  also  in  the 
way  of  excision.  Many  pages  of  ancestral  history  may  be  re- 
capitulated in  the  paragraphs  of  embryonic  development,  while 
others  may  not  be  so  much  as  mentioned.  And  that  this  is  the 
true  explanation  of  what  embryologists  term  the  '  direct '  develop- 
ment— or  of  a  more  or  less  sudden  leap  from  one  phase  to  another, 
without  any  appearance  of  intermediate  phases — is  proved  by  the 
fact  that  in  some  cases  both  direct  and  indirect  development  occur 
within  the  same  group  of  organisms,  some  genera  or  families 
having  dropped  out  the  intermediate  phases  which  other  genera 
or  families  retain." 


236  BIOLOGY:  GENERAL  AND  MEDICAL 

Minot  says,  "  The  embryo  is  not  a  correct  or  adequate 
record  of  the  ancestral  type, 

1.  Because   the   embryos   have   necessities   of 
their  own  which  have  led  to  their  modifica- 
tion in  the  course  of  evolution; 

2.  Because  the  embryos  consist  of  undifferen- 
tiated  cells; 

3.  Because  the  embryo  at  each  stage  must  be 
regarded   as   the   mechanical   cause   of  the 
next  and  following  stages. 

However,  Minot  declares  these  to  be  objections  to 
the  theory  rather  than  to  the  facts  which  he  believes 
fully  justify  the  interpretation  that  ontogeny  does  re- 
capitulate the  phylogeny. 

Though  the  resemblances  between  embryos  become 
more  and  more  pronounced  as  we  follow  them  back 
toward  the  egg,  there  is  no  time  when  embryos  belonging 
to  widely  divergent  organsims  are  precisely  alike. 
Every  embryo,  at  every  stage  of  its  development,  is  an 
individual  of  the  particular  genus  and  species  to  which 
it  belongs  and  has  at  every  stage  peculiarities  which 
distinguish  it  from  every  other  genus  or  species.  It  is, 
however,  invariably  true  that  the  more  closely  the 
species  are  related,  the  greater  is  the  resemblance  of  their 
embryos  at  all  stages,  and  the  more  widely  they  are 
separated  the  further  back  it  is  necessary  to  go  to  find 
the  phylogenetic  resemblances. 

If  we  examine  the  developing  mammalian  embryo 
for  phylogenetic  resemblances,  they  are  easily  found. 
Every  embryo  begins  by  segmentation  resulting  in  some 
kind  of  a  morula;  the  morula  always  passes  into  some 
kind  of  a  blastula  stage,  and  the  blastula  always  under- 
goes some  kind  of  gastrulation.  The  beginning  embryo 
is  always  elongate  and  slender.  The  gut  is  always 
formed  by  concrescence  of  the  folded  entoderm  and 
inclosed  by  concrescence  of  the  externally  folded  ecto- 
derm. The  vertebrate  embryo  diverges  from  all  others 
by  the  preponderating  importance  of  its  nervous  sys- 


ONTOGENESIS 


237 


tern  and  eyes,  provision  for  which  is  made  very  early, 
at  which  time  all  vertebrate  embryos  look  much 
alike.  All  of  the  vertebrate  embryos  have  long  tails 
(see  the  Romanes  diagrams).  The  ventral  surface 
becomes  closed  by  the  concrescence  of  buds  that  form 
the  face  and  neck  and  the  thoracic  and  abdominal  walls. 
The  branchial  region  of  all  vertebrate  embryos  show 
slits  or  folds  where  the  gills  of  the  fishes  and  batrachians 


FIG.  98. — Diagrams  illustrating  arrangement  of  primitive  heart  and  aortic 
arches  in  the  fiuman  embryo.  By  comparing  these  with  the  diagrams  showing 
the  increasing  complexity  of  the  heart  in  phylogenetic  development  (Chapter 
VII)  it  will  be  seen  that  in  the  development  of  the  human  heart  the  ontogeny, 
repeats  the  phylogeny.  (Modified  from  Allen  Thomson.) 


are  formed,  though  the  mammalian  embryos  do  not 
have  real  gill  clefts  at  this  time.  In  all  cases  the  limbs 
first  appear  as  buds  that  grow  into  shapeless  excrescences, 
which  subsequently  elongate,  differentiate,  and  become 
perfected,  it  being  impossible  to  tell  the  final  character 
of  these  members  for  some  time  after  they  have  first 
appeared. 

The  tail  persists  for  a  surprisingly  long  time  in  the 


238  BIOLOGY:  GENERAL  AND  MEDICAL 

human  embryo  (twenty-five  days)  and  gives  it  a  striking 
resemblance  to  the  embryos  of  lower  animals. 

The  heart  makes  its  appearance  as  a  simple  straight 
tube  similar  to  that  of  the  invertebrates,  becomes 
separated  into  an  anterior  ventricle  and  posterior  auricle 
like  that  of  the  fishes.  Later  it  consists  of  a  curved 
organ  with  two  incompletely  separated  auricles  and  one 
ventricle,  not  unlike  that  of  the  batrachians.  Much 
later  it  differentiates  into  the  four-chambered  viscus  of 
the  higher  vertebrates. 

Not  only  does  the  development  of  the  heart  thus 
conform  to  its  phylogeny,  but  the  development  of  the 
whole  circulatory  system  coincides  as  well.  The  heart 
and  great  vessels  first  appear,  the  small  vessels  and 
capillaries  later.  Further,  the  arrangement  of  the 
arteries  at  first  conforms  with  fair  accuracy  to  that 
seen  in  fish,  then  to  that  in  the  batrachians,  then  to 
that  of  mammals  as  can  at  once  be  seen  by  comparing 
the  figures  showing  the  embryology  of  the  human  heart 
and  vessels  with  those  of  the  different  phylogenetic 
groups. 

It  has  been  shown  that  the  first  separation  of  the 
egg  into  two  blastomeres  is  accompanied  by  a  fair 
uniformity  in  the  developmental  power  of  each,  so  that 
if  separated  at  that  time,  two  embryos  of  small  size 
may  develop.  Accidental  separation  of  the  primary 
blastomeres  seems  at  times  to  occur  among  the  highest 
animals,  and  it  is  probably  in  this  manner  that  homol- 
ogous twins  arise.  Such  twins  are  always  of  the  same 
sex;  resemble  each  other  so  closely  that  throughout  life 
they  are  frequently  mistaken  one  for  the  other;  possess 
the  same  general  tendencies  of  mind  and  body;  are 
predisposed  to  the  same  diseases;  attain  to  about  the 
same  general  intellectual  development,  and  not  infre- 
quently die  within  a  short  time  of  each  other,  some- 
times from  the  same  cause.  Galton's  studies  of  homolo- 
gous twins,  in  his  book  upon  "  The  Human  Faculty, "  are 
most  interesting  as  showing  how  completely  the  homol- 
ogous twins  are  identified. 


ONTOGENESIS  239 

As  such  twin  embryos  are,  during  their  embryonal 
development,  in  close  proximity  to  one  another,  there 
seems  to  be  an  occasional  tendency  for  the  growing  cells 
of  one  embryo  to  become  confused  with  those  of  its 
neighbor,  with  interesting  resulting  malformations. 
Thus,  one-half  may  grow  rapidly,  outstrip  and  include 
the  other,  whose  growth  is  consequently  disturbed  and 
inhibited,  so  that  one  foatus  with  normal  external  con- 
figuration, but  with  internal  confusion  explainable  on  no 
other  hypothesis,  may  arise. 

Or  the  foetuses  may  be  equal  or  nearly  equal  in  size, 
but  blended  or  connected  throughout,  or  at  the  cephalic, 
thoracic,  or  pelvic  portions,  sometimes  face  to  face,  some- 
times side  to  side,  sometimes  back  to  back.  The  rela- 
tive position  of  the  conjoined  parts  is  usually  normal 
— i.e.,  the  cephalic  and  caudal  ends  of  the  embryos 
correspond.  Sometimes,  however,  they  rotate  until 
the  cephalic  ends  are  opposed  and  the  caudal  ends 
conjoined,  or  the  caudal  ends  opposed  and  the  cephalic 
ends  conjoined. 

In  rare  cases  the  relation  of  one  embryonal  axis  to 
the  other  is  lost  and  the  attachment  of  one  embryo 
to  the  other  without  correspondence  of  the  parts. 

The  attachment  of  one  individual  to  the  other  may 
embrace  the  fundamental  and  vital  organs — heart, 
brain,  spinal  cord,  etc.;  or  may  be  through  compara- 
tively unimportant  structures,  the  twins  being  conjoined 
by  a  kind  of  slender  pedicle,  as  in  the  Siamese  twins. 

Some  knowledge  of  embryology  likewise  enables  one 
to  understand  certain  monstrous  formations  arising  in 
single  individuals.  Thus  the  bridging  of  the  neural 
canal  to  form  the  spinal  canal  in  which  the  future  spinal 
cord  is  to  lie,  is  one  of  the  first  features  of  mammalian 
development.  If  this  process  fail  anteriorly,  the  im- 
perfect covering  permits  morbid  changes,  known  as 
craniorrachischisis,  with  meningocele  or  encephalomen- 
ingocele,  and  terminating  in  anencephaly;  when  occur- 
ring posteriorly,  in  my elo  meningocele  and  spina  bifida. 


240  BIOLOGY:  GENERAL  AND  MEDICAL 

Partial  failure  of  the  anterior  concrescenses  by  which 
the  face  is  formed  explains  the  occurrence  of  hare-lip 
and  cleft  palate  and  similar  incomplete  fusion  of  the 
splanchnopleures  and  of  the  folds  which  form  the  geni- 
talia,  the  occurrence  of  extrophy  of  the  bladder,  epis- 
padias  and  hypospadia. 

The  rare  cases  of  reversed  viscera,  in  which  the  heart 
is  in  the  right  side,  the  liver  in  the  left,  and  the  spleen 
in  the  right — situs  inversus  viscerum — are  so  perfect  in 
detail  of  structure,  apart  from  the  reversed  condition, 
that  they  cannot  be  enumerated  among  the  monstrosi- 
ties, but  must  be  looked  upon  as  referable  to  occasional 
differences  in  the  right  or  left  impulse  of  growth  in  the 
respective  eggs.  This  coincides  with  Conklin's  observa- 
tions upon  snails'  eggs. 

Sex  Determination. — A  peculiarity  of  development  to 
which  no  reference  has  thus  far  been  made  is  that  of  the 
sex  of  the  individual,  and  as  it  is  of  importance  for  the 
safety  of  the  species  that  the  number  of  individuals  of 
each  sex  be  properly  proportioned,  it  becomes  a  matter 
of  considerable  interest  to  discover,  if  possible,  how  the 
sex  of  each  individual  is  determined. 

The  question  has  received  attention  from  early  times, 
and  the  number  of  theoretical  explanations  that  have  been 
suggested  corresponds  with  the  obscurity  and  difficulty 
of  the  problem.  "Blumenbach,  in  his  fascinating  brochure, 
'Ueber  den  Bildungstrieb,'  points  out  that  Drelincourt 
brought  together  two  hundred  and  sixty-two  groundless 
hypotheses  of  sex,  that  had  been  proposed,  and  Blumen- 
bach remarks,  quaintly,  that  nothing  is  more  certain  than 
that  Drelincourt's  own  theory  made  the  two  hundred 
and  sixty-third." 

At  the  present  time,  through  cytological  studies,  we  are 
no  doubt  much  nearer  to  the  truth  than  at  any  time  in 
the  past,  though  we  are  still  unable  to  formulate  a  theory 
of  sex  determination  that  is  generally  applicable. 

An  analysis  of  the  writings  upon  the  subject  may  be 
briefly  synoptized  as  follows: 


ONTOGENESIS  241 

I.  Theories    diminishing    in    validity  with    increasing 

knowledge  of  cytological  science. 
1.  That  sex  determination  depends  upon  conditions 
entirely  apart  from  the  germinal  cells. 

A.  That  the  sex  of  the  offspring  is  determined  by 

the  condition  of  the  parents: 
(a)  By  the  age  of  the  parents. — Sadler  taught 
that  the  sex  of  the  offspring  was  in  large  meas- 
ure determined  by  the  age  of  the  father.  He 
collected  statistics  that  seemed  to  show  that 
the  older  the  male  parent  was  the  greater  the 
number  of  male  offspring  he  produced. 
(6)  By  the  vigor  of  the  parents. — Popular  belief, 
said  to  be  based  upon  results  obtained  in  breed- 
ing domestic  animals,  has  led  to  the  opinion 
that  the  sex  of  the  offspring  corresponds  to  that 
of  the  less  vigorous  parent.  Almost  as  many 
facts,  however,  favor  the  opposite  opinion,  that 
it  is  the  more  vigorous  parent  that  determines 
the  sex  of  the  offspring. 

(c)  By  the  nutrition  of  the  parents. — The  whilom 
popular  theory  of  Schenk  taught  that  the 
nutrition  of  the  mother  was  at  the  foundation 
of  sex  determination.  If,  during  pregnancy, 
she  was  kept  in  the  highest  state  of  nutrition, 
female  offspring  were  more  numerous;  if,  on 
the  other  hand,  her  nutrition  was  kept  below 
par,  there  was  greater  likelihood  of  male  off- 
spring. 

In  all  three  cases  it  would  seem  as  though  the 
circumstance  of  the  occurrence  of  both  sexes 
in  cases  of  plural  births  would  overthrow  the 
validity  of  the  theory.  Thus  twins  are  fre- 
quently of  both  sexes,  and,  among  animals 
simultaneously  giving  birth  to  many  offspring 
at  the  same  time,  the  sexes  are  usually  almost 
equally  divided.  Moreover,  these  theories 
could  in  no  way  be  made  to  apply  in  the  cases 

16 


242  BIOLOGY:  GENERAL  AND  MEDICAL 

of  birds,  fishes,  insects,  and  still  more  lowly 

creatures,  where  the  eggs  leave  the  body  of  the 

mother  and  cannot  be  subsequently  influenced 

by  her. 

B.  That  the  sex  of  the  individual  is  determined  by 
the  nutritive  conditions  of  early  embryonal  life. 
These  theories  seem  to  be  based  upon  the  assump- 
tion that  every  organism  is  either  a  sexual  neuter 
or  a  hermaphrodite  during  early  embryonal  de- 
velopment, and  that  sex  makes  its  appearance  or 
one  or  the  other  sex  succeeds  in  preponderating 
as  development  advances.  Most  of  the  investi- 
gations upon  this  phase  of  the  subject  have 
been  performed  upon  animals  with  a  prolonged 
period  of  embryonal — i.e.,  larval — life. 

Mrs.  Treat  experimented  with  caterpillars,  and 
found  that  when  they  were  half-starved  they 
subsequently  transformed  into  male  insects. 

E.  Young  found  that  tadpoles  kept  under 
normal  conditions  developed  into  sexually  perfect 
frogs  in  the  proportions  of  43  males  to  57  females. 
If,  however,  they  were  given  an  abundant  flesh 
diet,  the  percentage  of  males  was  greatly  in- 
creased. 

It  is  well  known  that  aphides  produce  only 
females  during  the  summer  months  when  the 
conditions  of  nutrition  are  good,  but  also  produce 
males  when  the  less  favorable  conditions  of 
autumn  come  on. 

Maupas  and  others  have  found  that  rotiers  and 
certain  crustaceans  appear  to  produce  excessive 
numbers  of  females  in  the  presence  of  abundant 
nutrition. 

Careful  consideration  of  these  findings  will 
show  that  in  every  case  the  conclusions  are  based 
upon  some  kind  of  misinterpretation  of  the  facts, 
and  in  most  of  them  the  factor  of  selective  mor- 
tality has  not  been  given  proper  attention. 


ONTOGENESIS  243 

C.  That  sex-determination  depends  upon  the  age 
of  the  ovum  at  the  time  of  fertilization. 

Thury  (1863)  and  Busing  (1883)  taught  that 
when  the  ovum  was  fertilized  soon  after  ovula- 
tion,  it  developed  into  a  female;  if  long  after 
ovulation,  into  a  male. 

Hensen  was  of  the  opinion  that  females  re- 
sulted when  both  ova  and  spermatozoa  were  in 
the  most  active  condition  at  the  time  of  fertiliza- 
tion. 

Vernon,  in  cross  fertilizing  certain  echinoderms, 
thought  that  the  fresher  gamete  exerted  the 
greater  influence  in  sex  determination. 

D.  That  sex  determination  depends  upon  certain 
conditions  of  fertilization. 

Canestrini  long  ago  suggested  that  the  sex  of 
the  individual  was  determined  by  the  number  of 
spermatozoa  that  enter  the  ovum  at  the  time  of 
fertilization.  But  as  it  is  now  well  known  that 
only  one  spermatozoon  enters  to  fertilize  the 
ovum,  of  course  this  theory  fails. 

That  conditions  of  fertilization  do  have  some- 
thing to  do  with  sex  determination  is,  however, 
quite  certain.  Thus,  in  bees  .the  spermatozoa 
of  the  male  are  contained  sometimes  during  the 
several  years  of  the  life  of  the  queen,  in  living 
condition,  in  a  special  receptacle  made  to  receive 
them,  from  which  they  can  be  liberated  to  fertil- 
ize the  eggs  or  not,  at  the  will  of  the  queen. 
Ordinarily  the  eggs  are  fertilized  and  all  give  rise 
to  females,  but  unfertilized  eggs  all  develop  into 
males. 

II.  Theories    increasing    in    validity    with    increasing 
knovledge  of  cytological  science: 

E.  That  the  sex  of  the  individual  is  in  every  case 
predetermined  in  the  ovum. 

This  must  not  be  construed  to  mean  that  one 
ovary  produces  male  eggs  and  the  other  female 


244  BIOLOGY:  GENERAL  AND  MEDICAL 

eggs,  though  such  a  theory  has  been  held  and 
taught.  There  is,  however,  no  doubt  but  that 
some  organisms,  especially  insects,  produce  eggs, 
some  of  which  are  male  and  some  female  in  qual- 
ity, having  morphological  differences  by  which 
they  may  be  recognized. 

F.  That  the  sex  of  the  offspring  is  determined  by 
the  spermatozoa.     In  its  original  form  this  idea 
had  no  scientific  foundation,  for  it  was  supposed 
that  the  spermatozoa  from  one  testis  gave  rise  to 
male,  those  from  the  other  to  female  offspring. 

It  began,  however,  to  have  significance  in  a 
quite  different  sense,  as  a  result  of  recent  scien- 
tific investigation  of  the  germinal  cells  and  the 
mechanism  of  fertilization.  Thus  investigations 
of  parthenogenesis  showed  that  unfertilized 
eggs  always  develop  into  organisms  of  one  sex, 
while  fertilized  eggs  might  develop  into  those  of 
either  sex. 

Further  significance  arose  from  the  discovery 
by  Henking  that  certain  insects  like  Pyrorchis 
produce  two  kinds  of  spermatozoa  in  equa) 
numbers.  Fertilization  by  spermatozoa  of  one 
kind  gave  rise  to  organisms  of  one  sex,  fertiliza- 
tion by  spermatozoa  of  the  other  kind  to  the 
opposite  sex.  These  facts  were  confirmed  by 
Paulinier,  and  are  now  known  to  be  true  of  more 
than  one  hundred  different  kinds  of  insects. 

G.  That  the  sex  of  the  offspring  is  an  accident  of 
fertilization.     This  hypothesis  is  based  upon  the 
discovery    in    1908,  by  Clarence    E.  McClung, 
that  the  spermatocytes  of  certain  insects  con- 
tain   an    accessory    or    unpaired     chromosome, 
whose  passage  into  the  ovum  during  the  process 
of  fertilization  determines  the  sex  of  the  future 
product  of  the  fertilization.    E.  B.  Wilson  quickly 
realized  the  importance  of  this  odd  chromosome, 
which  has  now  been  located  in  the  spermatozoa 
of  more  than  a  hundred  different  kinds  of  in- 
sects, arachnids,  myriapods,  etc. 


ONTOGENESIS  245 

In  all'  of  these  cases  the  spermatozoa  are 
formed  in  pairs,  the  sperm  mother-cell  that  gives 
rise  to  each  pair  manifesting  the  ordinary  mode 
of  nuclear  division  with  paired  chromosomes, 
one  member  of  each  pair  passing  into  each  sper- 
matozoon. In  addition  to  these,  however,  the 
sperm  mother-cell  contains  an  unpaired  element, 
sometimes  consisting  of  a  large  chromosome, 
sometimes  of  a  group  of  peculiar  chromosomes, 
which  pass  into  one  or  the  other  of  the  sperma- 
tozoa. Such  elements  Wilson  calls  the  x-ele- 
ments  or  heterochromosomes. 

According  to  the  researches  of  McClung  and 
Wilson,  now  well  confirmed  by  many  others,  it 

Spermatocyte  Division  Zygotes 


FIG.  99. — Accessory  or  z-chromosome  in  Anosa.  E.  B.  Wilson — Recent  Re- 
searches on  the  Determination  and  Heredity  of  Sex-Science,  Jan.  8,  1909. 

Sex  determined  by  the  x-element.  Equally  paired  chromosome  =  ?.  Un- 
equally =  cf.  i-chromosome  black. 


is  this  z-element  that  determines  the  sex  of  the 
offspring.  Eggs  fertilized  by  spermatozoa  con- 
taining the  z-element  or  heterochromosome,  be- 
come females;  those  fertilized  by  spermatozoa 
not  containing  these  elements,  males. 

In  certain  cases — bees — the  spermatozoa  not 
containing  the  ^-elements  all  die  and  degenerate, 
hence  in  these  insects  all  fertilized  eggs  must  be 
females,  while  eggs  not  fertilized  become  males. 

The  trend  of  modern  cytologists  is  in  favor 
of  the  view  that  the  x-element  is  the  sex  deter- 


246  BIOLOGY:  GENERAL  AND  MEDICAL 

minant,  or  at  least  is  the  sex  index.  The  differ- 
ence between  the  male  and  female  organism  is 
that  the  male  comes  from  an  egg  which  develop- 
ing either  parthenogenetically  or  after  fertiliza- 
tion contains  only  a  single  z-element,  while  the 
female  starts  from  one  which  developing  either 
parthenogenetically  or  after  fertilization  contains 
two  rr-elements. 

The  ovum  of  a  sexual  egg  in  the  process  of 
maturation  discards  half  of  its  normal  comple- 
ment of  the  rc-element.  If  it  be  subsequently 
fertilized  by  a  spermatozoon  containing  an  re- 
element,  these  elements  being  paired,  the  result- 
ing organism  is  a  female;  if  by  a  spermatozoon 
devoid  of  an  ^-element,  the  elements  being  un- 
paired, into  a  male. 

H.  That  the  sex  of  the  offspring  is  a  Mendelian 
character  but  still  an  accident  of  fertilization. 
To  fully  comprehend  this  theory  it  is  necessary 
that  the  reader  shall  read  the  theory  of  Mendel 
as  outlined  in  the  chapter  upon  "Conformity  to 
Type." 

Castle,  Correns,  Bateson,  Doncaster,  and  others 
have  written  in  support  of  this  view,  and  their 
theories  are  ingenious  and  suggestive. 

Castle  believes  that  both  male  and  female 
organisms  are  Mendelian  hybrids  (male-female 
hybrids),  maleness  and  femaleness  being  Mendel- 
ian characters,  with  respective  male  and  female 
dominance.  During  the  maturation  of  the 
germinal  cells  the  usual  disruption  of  character 
takes  place,  so  that  male  and  female  ova  and 
male  and  female  spermatozoa  are  produced.  He 
made  the  assumption  that  there  were  selections 
and  repulsions  in  fertilization  so  that  spermatozoa 
and  ova  bearing  the  same  sex  never  conjoined. 

Instead  of  regarding  both  male  and  female  as 
such,  Correns  looked  upon  the  male  as  the  sex 


ONTOGENESIS  247 

hybrid  and  the  female  as  a  pure  or  homozygous 
organism.  The  male  character  he  regarded  as 
dominant.  All  ova  were  unisexual,  and  hence 
always  female,  whether  fertilized  or  not.  The 
spermatozoa,  on  the  other  hand,  carried  either 
male  or  female  characters. 

If  the  female  ovum  was  fertilized  by  a  sperma- 
tozoon carrying  only  female  characters,  it,  of 
course,  remained  female;  but  if  it  were  fertilized 
by  a  spermatozoon  carrying  male  characters,  the 
male  character  always  being  dominant,  it,  of 
necessity,  became  a  male. 

The  theory  is  shown  to  be  false  by  remember- 
ing that  the  unfertilized  eggs  of  bees  develop  into 
males. 

The  reverse  aspect  of  the  case  might  overcome 
this  objection  as  Bateson  has  suggested.  Thus, 
it  might  be  the  female  that  is  the  hybrid  with 
femaleness  dominant.  Though  by  this  assump- 
tion we  are  relieved  of  the  dilemma  into  which  we 
were  thrown  by  the  peculiarity  of  the  bee's  eggs, 
we  fall  into  a  new  one,  because  of  the  observa- 
tions by  Henking,  McClung,  and  Wilson  that 
morphologically  different  spermatozoa  determine 
the  sex  of  many  kinds  of  insects,  and  because 
of  the  difficulty  of  explaining  many  of  the  cir- 
cumstances attending  parthenogenesis. 

A  very  ingenious  solution  of  the  difficulty  of 
avoiding  entanglements  in  endeavoring  to  ex- 
plain sex  along  Mendelian  lines  has  been  thought 
out  by  L.  Doncaster,  who  suggests  that  the 
Mendelian  pairs  are  not  male  and  female,  but  are 
male  and  no  sex  and  female  and  no  sex.  The 
male  is  pure,  but  produces  spermatozoa  of  two 
kinds,  viz.,  those  with  sex  determinants  and  those 
without  them.  The  female  is  a  sex  hybrid  pro- 
ducing both  male  and  female  eggs  in  equal  num- 
bers. He  assumes  that  there  is  selective  fertiliza- 
tion by  which  female  eggs  fertilized  by  male 


248  BIOLOGY:  GENERAL  AND  MEDICAL 

spermatozoa  give  rise  to  females  and  male  eggs 
fertilized  by  no  sex  spermatozoa  give  rise  to 
males. 

In  accounting  for  the  conditions  of  partheno- 
genesis, it  is  assumed  that  females  are  of  two 
kinds  as  the  result  of  fertilization  by  different 
kinds  of  spermatozoa,  and  that  their  maturation 
processes  differ  so  that  they  may  give  rise  to 
either  males  or  females.    These  views  accord  with 
certain  observations  made  by  Doncaster  upon  the 
maturation  phenomena  of  insects. 
From  these  brief  outlines  of  the  theoretical  aspects  of 
the  problem,  it  will  appear  that  the  matter  of  sex  deter- 
mination is  by  no  means  clear,  and  that  there  are  diffi- 
culties in  the  way. of  solving  it. 

The  theories  most  concordant  with  modern  scientific 
information  are  McClung's  theory  of  the  z-element  and 
the  Mendelian  theory.  These  are  not  discordant  the- 
ories, and  both  may  contain  the  truth;  the  former  is  a 
qualitative,  the  latter  a  quantitative  demonstration  of 
what  may  take  place. 


REFERENCES. 

O.  HERTWIG:  "Text-book  of  the  Embryology  of  Man  and  Mam- 
mals." Translated  by  E.  L.  Mark,  N.  Y. 

JOHN  C.  HEISLER:     "A  Text-book  of  Embryology,"  Phila. 

L.  P.  McMuRRicn:  "The  Development  of  the  Human  Body," 
Philadelphia,  1904. 

C.  S.  MINOT:  "A  Laboratory  Text-book  of  Embryology,"  Phila., 
1903. 

A.  M.  MARSHALL:     "The  Frog, "  etc.,  N.  Y.,  1906. 

C.  E.  McCLUNG:  The  Biological  Bulletin,  1902,  iii,  43. 

E.  B.  WILSON:  "Recent  Researches  on  the  Determination  and 
Heredity  of  Sex,"  1909. 

The  Encyclopedia  Britannica:  Article  on  Sex. 

T.  H.  MORGAN:  "Experimental  Zoology."  The  Biological  Bulletin, 
1902,  iii,  56. 

L.  DONCASTER:  "Determination  of  Sex,"  Cambridge  University 
Press,  1914. 


CHAPTER  X. 
CONFORMITY  TO  TYPE. 

There  are  few  facts  better  recognized,  yet  less  under- 
stood, than  that  the  offspring  resembles  its  parents. 
Such  resemblance  is  said  to  be  inherited,  and  the 
qualities  by  which  it  is  brought  about,  hereditary. 
The  faithful  reappearance  of  these  qualities  not  only 
causes  each  individual  to  resemble  its  parents,  but  also 
to  conform  to  the  type  of  its  species.  Failure  in  their 
appearance  may  lead  to  a  divergence  from  the  parent  type 
and  may  result  in  the  development  of  new  species. 
Heredity,  therefore,  is  of  far-reaching  importance,  for 
it  not  only  determines  ontogeny,  but  also  phylogeny. 

To  those  who  have  become  reasonably  familiar  with 
the  reproductive  processes  it  may  not  be  surprising 
that  the  unicellular  organisms  conform  to  their  specific 
types,  seeing  that  each  is  but  a  portion  of  a  preexisting 
organism  whose  entire  reproductive  energy  at  the  time 
of  multiplication  has  been  directed  toward  securing  for 
each  resulting  half  an  exactly  equal  quantity  of  parental 
substance.  Nor  is  it  surprising  that  a  bud  from  a 
hydra  should  eventually  come  to  resemble  the  hydra, 
seeing  that  any  considerable  part  cut  off  from  the  hydra 
eventually  regenerates  so  as  to  return  to  parental 
resemblance.  But  no  amount  of  familiarity  with  the 
phenomena  can  make  the  thoughtful  mind  cease  to 
wonder  when  he  sees  a  tiny  undifferentiated  spore  grow 
into  a  beautiful  fern  by  which  myriads  of  new  spores 
will  be  produced,  a  very  simple  seed  grow  into  a  great 
tree  covered  with  leaves  and  flowers,  or  an  egg  trans- 
formed without  external  assistance  into  a  living,  active 
bird  covered  with  feathers.  That  so  much  should  be 
potentially  enclosed  in  a  single  cell,  seems  impossible! 
249 


250  BIOLOGY:  GENERAL  AND  MEDICAL 

What  is  in  the  egg?  What  is  the  nature  of  these 
maternal  and  paternal  influences?  How  can  they  deter- 
mine phylogeny  and  ontogeny?  How  can  they  deter- 
mine every  detail  of  the  new  being  from  its  general 
configuration  to  the  finest  details  of  internal  structure; 
from  the  color  patterns  upon  its  smallest  feathers  to  the 
choice  of  its  food,  the  quality  of  its  voice,  or  its  future 
habits  of  roosting  in  trees  or  building  its  nest  on  the 
ground? 

Yet  all  this  and  more  is  accomplished  without  any 
outside  influence  except  the  degree  of  warmth  required 
for  the  incubation.  All  of  the  forces  that  effect  these 
results  are  intrinsic  in  the  egg,  yet  without  any  visible 
explanation.  Indeed  some  of  the  influences  resident  in 
eggs  do  not  appear  to  become  active  for  years  after 
the  adult  individual  has  formed.  Thus  among  human 
beings  we  find  it  to  be  predetermined  in  the  egg  that 
one  shall  lose  his  hair  early  or  late,  have  his  hair 
whiten  early  or  late,  tend  to  corpulence  at  a  certain  age, 
and  even  tend  to  die  of  apoplexy  when  a  certain  age  is 
reached.  It  seems,  therefore,  as  though  eggs  are  charged 
with  impulses  so  numerous,  so  diversified,  and  so  persist- 
ent as  to  determine  the  entire  future  of  the  individual 
and  leave  nothing  to  chance  or  to  circumstance. 

It  is  small  wonder  that  matters  of  such  surpassing 
interest  should  have  proved  a  fascinating  study  to 
thinking  men  in  all  departments  of  learning  and  that 
many  theories  should  have  been  devised  for  their 
explanation. 

Herbert  Spencer1  conceived  that  the  form  of  each 
living  creature  was  determined  by  a  "  peculiarity  in  the 
constitution  of  its  physiological  units,  that  these  have 
a  special  structure  in  which  they  tend  to  arrange  them- 
selves, just  as  have  the  simpler  units  of  inorganic 
matter."  .  .  . 

"  We  must  conclude  that  the  likeness  of  any  organism  to  either 
parent  is  conveyed  by  the  special  tendencies  of  its  physiological 
1  "Principles  of  Biology,"  I,  p.  254,  1866. 


CONFORMITY   TO   TYPE  251 

units  derived  from  that  parent.  In  the  fertilized  germ  we  have  two 
groups  of  physiological  units,  slightly  different  in  their  structures. 
These  slightly  different  units  severally  multiply  at  the  expense 
of  the  nutriment  supplied  to  the  unfolding  germ,  each  kind  mould- 
ing this  nutriment  into  units  of  its  own  type.  Throughout  the 
process  of  evolution,'  the  two  kinds  of  units,  mainly  agreeing  in 
their  polarities  and  the  form  which  they  tend  to  build  themselves 
into,  but  having  minor  differences,  work  in  unison  to  produce  an 
organism  of  the  species  from  which  they  were  derived,  but  work  in 
antagonism  to  produce  copies  of  their  respective  parent  organisms. 
And  hence  ultimately  results  an  organism  in  which  the  traits  of  the 
one  are  mixed  with  traits  of  the  other." 

"Quite  in  harmony  with  this  conclusion  are  certain  implica- 
tions .  .  .  noticed  respecting  the  characters  of  sperm  cells  and 
germ  cells.  We  saw  sundry  reasons  for  rejecting  the  supposition 
that  they  are  highly  specialized  cells  and  for  accepting  the  opposite 
supposition  that  they  are  cells  differing  from  the  others  rather  in 
being  unspecialized.  And  here  the  assumption  to  which  we  seem 
driven  by  the  ensemble  of  the  evidence  is,  that  the  sperm  cells  and 
germ  cells  are  essentially  nothing  more  than  the  vehicles,  in  which 
are  contained  small  groups  of  physiological  units  in  a  fit  state  for 
obeying  their  proclivity  toward  the  structural  arrangement  of  the 
species  they  belong  to." 

These  "units"  of  which  Spencer  speaks  are  regarded  as 
intermediate  between  the  chemical  units  or  molecules  and 
the  morphological  units  or  cells.  They  must  be  immensely 
more  complicated  than  the  chemical  units,  and  must, 
therefore,  correspond  to  groups  of  molecules.  The  whole 
organism  is  supposed  to  be  composed  of  them,  all  alike  in 
kind.  The  germ  cells  contain  small  groups  of  them. 

"The  former  supposition  makes  regeneration  possible  to  each  suf- 
ficiently large  portion  of  the  body,  while  the  latter  gives  the  germ  cell 
the  power  of  reproducing  the  whole;  inasmuch  as  the  'polarity'  of 
the  'units'  leads  to  their  arrangement  in  such  a  way  that  the  whole 
'crystal,'  the  organism,  is  restored  or  even  formed  anew.^  The  mere 
difference  in  the  arrangement  of  the  units  alike  in  kind  determines  the 
diversity  of  the  parts  of  the  body,  while  the  distinction  between 
different  species  and  that  between  different  individuals  is  due  to  a 
diversity  in  the  constitution  of  the  units." 

Weismann,  in  considering  Spencer's  theory,  says  that 
"the  assumption  of  these  'physiological  units'  does 
not  suffice  as  an  interpretation  of  heredity;  it  proves  in- 
sufficient even  in  interpreting  the  differentiation  of  or- 
gans in  simple  autogeny,  quite  apart  from  the  question  of 
amphigonic  heredity.  But  it  has  the  merit  of  having 
utilized  the  smallest  vital  particles  as  constituent  ele- 


252  BIOLOGY;  GENERAL  AND  MEDICAL 

ments  of  the  organism  and  of  having  made  them  the 
basis  of  a  theory  of  heredity." 

Darwin1  formulated  a  theory  of  inheritance  which  he 
at  first  imagined  to  correspond  fairly  well  with  Spencer's, 
though  important  differences  were  subsequently  pointed 
out.  This  theory  he  calls  pangenesis  and  explains  as 
follows: 

"It  is  universally  admitted  that  the  cells  or  units  of  the  body 
increase  by  self -division  or  proliferation,  retaining  the  same  nature, 
and  that  they  ultimately  become  converted  into  various  tissues 
and  substances  of  the  body.  But  besides  this  means  of  increase  I 
assume  that  the  units  throw  off  minute  granules  which  are  dispersed 
throughout  the  whole  system;  that  these,  when  supplied  with 
proper  nutriment,  multiply  by  self-division,  and  are  ultimately 
developed  into  units  like  those  from  which  they  were  originally 
derived.  These  granules  may  be  called  gemmules.  They  are 
collected  from  all  parts  of  the  system  to  constitute  the  sexual 
elements,  and  their  development  in  the  next  generation  forms  a 
new  being;  but  they  are  likewise  capable  of  transmission  in  a  dor- 
mant state  to  future  generations  and  may  then  be  developed. 
Their  development  depends  upon  their  union  with  other  partially 
developed  or  nascent  cells  which  precede  them  in  the  regular 
course  of  growth.  .  .  .  Gemmules  are  supposed  to  be  thrown 
off  by  every  unit,  not  only  during  the  adult  state,  but  during  each 
stage  of  development  of  every  organism,  but  not  necessarily  during 
the  continued  existence  of  the  same  unit.  Lastly,  I  assume  that 
the  gemmules  in  their  dormant  state  have  a  mutual  affinity  for 
one  another,  leading  to  their  aggregation  into  buds  or  into  the 
sexual  elements.  Hence  it  is  not  the  reproductive  organs  or  buds 
which  generate  into  new  organisms,  but  the  units  of  which  each 
individual  is  composed.  These  assumptions  constitute  the 
provisional  hypothesis  which  I  have  called  pangenesis." 

After  a  lengthy  application  of  the  theory  to  the  facts 
to  be  explained,  he  concludes  as  follows: 

"The  hypothesis  of  pangenesis,  as  applied  to  the  several  great 
classes  of  facts  just  discussed,  no  doubt  is  extremely  complex,  but 
so  are  the  facts.  The  chief  assumption  is  that  all  the  units  of  the 
body,  besides  having  the  universally  admitted  power  of  growing 
by  self-division,  throw  off  minute  gemmules  which  are  dispersed 
throughout  the  system.  Nor  can  this  assumption  be  considered 
too  bold,  for  we  know  from  the  cases  of  graft-hybridization  that 
formative  matter  of  some  kind  is  present  in  the  tissues  of  plants, 
which  is  capable  of  combining  with  that  included  in  another 
individual,  and  of  producing  every  unit  of  the  whole  organism. 
But  we  have  further  to  assume  that  the  gemmules  grow,  multiply, 

'"The  Variation  of  Animate  and  Plants  under  Domestication,"  1868,  II, 
Chapter  XXVII,  p.  349. 


CONFORMITY   TO   TYPE  253 

and  aggregate  themselves  into  buds  and  the  sexual  elements; 
their  development  depending  on  their  union  with  other  nascent 
cells  or  units.  They  are  also  believed  to  be  capable  of  trans- 
mission in  a  dormant  state,  like  seeds  in  the  ground,  to  successive 
generations." 

"In  a  highly  organized  animal,  the  gemmules  thrown  off  from 
each  different  unit  throughout  the  body  must  be  inconceivably 
numerous  and  minute.  Each  unit  of  each  part,  as  it  changes 
during  development,  and  we  know  that  some  insects  undergo  at 
least  twenty  metamorphoses,  must  throw  off  its  gemmules.  But 
the  same  cells  may  long  continue  to  increase  by  self-division,  and 
even  become  modified  by  absorbing  peculiar  nutriment,  without 
necessarily  throwing  off  modified  gemmules." 

"  All  organic  beings,  moreover,  include  many  dormant  gemmules 
derived  from  their  grandparents  and  more  remote  progenitors, 
but  not  from  all  their  progenitors.  These  almost  infinitely  numer- 
ous and  minute  gemmules  are  contained  within  each  bud,  ovule, 
spermatozoon,  and  pollen  grain.  Such  an  admission  will  be 
declared  impossible;  but  number  and  size  are  only  relative  diffi- 
culties. Independent  organisms  exist  which  are  barely  visible 
under  the  highest  powers  of  the  microscope,  and  their  germs  must 
be  exceedingly  minute.  Particles  of  infectious  matter,  so  small 
as  to  be  wafted  by  the  wind  or  to  adhere  to  smooth  paper,  will 
multiply  so  rapidly  as  to  infect  within  a  short  time  the  whole  body 
of  a  large  animal.  We  should  also  reflect  on  the  admitted  number 
and  minuteness  of  the  molecules  composing  a  particle  of  ordinary 
matter.  The  difficulty,  therefore,  which  at  first  appears  insur- 
mountable, of  believing  in  the  existence  of  gemmules  so  numerous 
and  so  small  as  they  must  be  according  to  our  hypothesis  has  no 
great  weight.  The  units  of  the  body  are  generally  admitted  by 
physiologists  to  be  autonomous. 

"I  go  one  step  further  and  assume  that  they  throw  off  repro- 
ductive gemmules. 

"Thus  an  organism  does  not  generate  its  kind  as  a  whole,  but 
each  separate  unit  generates  its  kind.  It  has  often  been  said  by 
naturalists  that  each  cell  of  a  plant  has  the  potential  capacity 
of  reproducing  the  whole  plant;  but  it  has  this  power  only  in 
virtue  of  containing  gemmules  derived  from  every  part.  When  a 
cell  or  unit  is  from  some  cause  modified,  the  gemmules  derived 
from  it  will  be  in  like  manner  modified. 

"  If  our  hypothesis  be  provisionally  accepted,  we  must  look  at  all 
the  forms  of  asexual  reproduction,  whether  occurring  at  maturity 
or  during  youth,  as  fundamentally  the  same  and  dependent  on 
the  mutual  aggregation  and  multiplication  of  the  gemmules. 
The  regrowth  of  an  amputated  limb  and  the  healing  of  a  wound  is 
the  same  process  partially  carried  out.  Buds  apparently  include 
nascent  cells,  belonging  to  that  stage  of  development  at  which 
the  budding  occurs,  and  the  cells  are  ready  to  unite  with  the 
gemmules  derived  from  the  next  succeeding  cells. 

"The  sexual  elements,  on  the  other  hand,  do  not  include  such 
nascent  cells;  and  the  male  and  female  elements  taken  separately 
do  not  contain  a  sufficient  number  of  gemmules  for  independent 
development,  except  in  the  cases  of  parthenogenesis. 


254  BIOLOGY:  GENERAL  AND  MEDICAL 

"The  development  of  each  being,  including  all  the  forms  of 
metamorphosis  and  metagenesis,  depends  on  the  presence  of 
gemmules  thrown  off  at  each  period  of  life  and  on  their  develop- 
ment, at  a  corresponding  period,  in  union  with  preceding  cells, 
Such  cells  may  be  said  to  be  fertilized  by  the  gemmules  which 
come  next  in  due  order  of  development.  Thus  the  act  of  ordinary 
impregnation  and  the  development  of  each  part  in  each  being  are 
closely  analogous  processes.  The  child,  strictly  speaking,  does 
not  grow  into  the  man,  but  includes  germs  which  slowly  and 
successively  become  developed  and  form  the  man. 

"In  the  "child,  as  well  as  in  the  adult,  each  part  generates  the 
same  part.  Inheritance  must  be  looked  at  as  merely  a  form  of 
growth,  like  the  self-division  of  a  lowly  organized  unicellular 
organism.  Reversion  depends  on  the  transmission  from  the  fore- 
father to  his  descendants  of  dormant  gemmules,  which  occasionally 
become  developed  under  certain  known  or  unknown  conditions. 
Each  animal  and  plant  may  be  compared  with  a  soil  full  of  seeds, 
some  of  which  soon  germinate,  some  lie  dormant  for  a  period, 
whilst  others  perish.  When  we  hear  it  said  that  a  man  carries  in 
his  constitution  the  seeds  of  an  inherited  disease,  there  is  much 
truth  in  the  expression.  No  other  attempt,  as  far  as  I  am  aware, 
has  been  made,  imperfect  as  this  confessedly  is,  to  connect  under 
one  point  of  view  these  several  grand  classes  of  facts.  An  organic 
being  is  a  microcosm — a  little  universe,  formed  of  a  host  of  self- 
propagating  organisms,  inconceivably  minute  and  numerous  as 
the  stars  in  heaven." 

In  criticising  this  theory  of  pangenesis,  Galton1  points 
out  that  though  it  is  quite  in  accord  with  Darwin's 
theories  and  fully  accounts  for  such  features  as  are 
embraced  in  the  hereditary  transmission  of  those  char- 
acters upon  which  species  are  supposed  to  separate, 
there  are  certain  difficulties,  both  theoretical  and  prac- 
tical, in  the  way  of  its  acceptance.  Thus,  the  gemmules 
given  off  by  the  cells  must  be  looked  upon  as  of 
colloidal  nature  and  therefore  cannot  be  supposed  easily 
to  transfuse  through  membranes  as  their  free  circula- 
tion in  the  body  would  necessitate.  Being  in  large 
numbers  in  the  maternal  circulation,  they  must  easily 
find  their  way  into  the  foetal  circulation,  unduly  im- 
pressing the  offspring  with  maternal  material.  For  this 
reason,  the  offspring  should  much  more  closely  resemble 
the  maternal  grandmother  than  any  other  progenitor, 
which  is  certainly  not  the  case.  If  present  in  the  circu- 
lation, they  should  pass  from  one  animal  into  another 

i "  A  Theory  of  Heredity,"  Jour,  of  the  Anthropological  Institute,  V,  London. 
1876,  p.  329. 


CONFORMITY   TO   TYPE  255 

if  transfusion  of  blood  were  practised  and  then  should 
influence  the  germinal  cells  of  the  animal  into  which 
they  were  introduced.  To  determine  this  point,  Galton 
"  largely  transfused  the  blood  of  an  alien  species  of  rab- 
bit into  the  blood  vessels  of  male  and  female  silver-gray 
rabbits,  from  which  he  afterwards  bred."  "  He  repeated 
this  process  for  three  generations  and  found  not  the 
slightest  sign  of  any  deterioration  in  the  silver-gray 
breed."  Having  been  criticised  by  Darwin  for  the 
manner  in  which  the  experiments  were  performed,  he 
subsequently  repeated  them  with  improved  apparatus 
and  on  an  equally  large  scale  for  two  more  generations, 
but  without  differing  results. 

Galton  therefore  devised  a  new  theory  of  heredity — 
the  theory  of  the  stirp — based  upon  the  theory  of  pan- 
genesis,  but  differing  from  it  in  certain  essentials. 

The  term  "stirp,"  from  the  Latin  stirpes,  a  root, 
"  is  used  to  express  the  sum  total  of  the  germs,  gemmules, 
or  whatever  they  may  be  called,  which  are  to  be  found, 
according  to  every  theory  of  organic  units,  in  the  newly 
fertilized  ovum." 

The  theory  is  postulated  in  four  parts,  thus:  "  1.  Each 
of  the  enormous  number  of  quasi-independent  units  of 
which  the  body  consists  has  a  separate  origin  or  germ. 
2.  The  stirp  contains  a  host  of  germs,  much  greater  in 
number  and  variety  than  the  organic  units  of  the  bodily 
structure  that  is  about  to  be  derived  from  them,  so  that 
comparatively  few  individuals  out  of  the  host  of  germs 
achieve  development.  3.  The  undeveloped  germs  retain 
their  vitality  that  they  may  propagate  themselves  while 
still  in  the  latent  state  and  contribute  to  form  the  stirps 
of  the  offspring.  4.  Organization  wholly  depends  upon 
the  mutual  affinities  and  repulsions  of  the  separate 
germs:  first  in  their  earliest  stirpal  stage  and  subse- 
quently during  all  the  processes  of  development." 

"  It  is  thus  seen  that  the  stirp  itself  contains  all  of  the 
essential  units,  to  which  few  are  added — that  some  must 
circulate  and  be  added  to  .those  in  the  stirp  is  granted, 
and  they  explain  the  rare  cases  in  which  zebra  marks 


256  BIOLOGY:  GENERAL  AND  MEDICAL 

occur  on  the  foal  of  a  thoroughbred  mare  by  a  thorough- 
bred horse,  owing  to  the  fact  that  the  mare  had  pre- 
viously born  a  mule  to  a  zebra;  but  to  have  them  circu- 
late in  such  numbers  and  so  constantly  as  the  doctrine 
of  pangenesis  implies  would  over-explain  such  cases." 

"  Of  the  two  groups  of  germs,  the  one  consisting  of 
those  that  succeed  in  becoming  developed  and  in  form- 
ing the  bodily  structure,  and  the  other  consisting  of 
those  that  remain  continually  latent,  the  latter  vastly 
preponderates  in  number."  "  We  should  expect  the 
latent  germs  to  exercise  a  corresponding  predominance 
in  matters  of  heredity,  unless  it  can  be  shown  that,  on 
the  whole,  the  germ  that  is  developed  into  a  cell  be- 
comes thereby  more  fertile  than  if  it  had  remained  latent. ' ' 

The  theory  of  the  stirp  transfers  the  hereditary 
material  from  the  somatic  to  the  germinal  cells.  Con- 
trasting his  views  with  those  of  Darwin,  the  following 
language  is  used  by  Galton: 

1.  "There  are  cells  and  there  are  a  great  number  of  gemmules. 

2.  "The  cells  multiply  by  self-division,  and  during  this  process 
they  throw  off  gemmules.     (I  look  upon  this  process  of  throwing 
off  the  gemmules  to  be  of  such  minor  importance  as  to  have  no 
effect  whatever  upon  the  cases  we  have  thus  far  considered.     Its 
existence  is  granted,  but  only  as  a  subordinate  process,  to  account 
for  the  exceptional  cases  to  be  hereafter  considered  and  not  as 
the  primary  process  in  heredity.) 

3.  "The  gemmules  multiply  by  self-division,  and  any  gemmule 
admits  under  favorable  circumstances  of  being  developed  into  a 
cell.     (I  look  upon  this  as  the  primary  process  in  heredity.) 

4.  "The  personal  structure  is  formed  by  a  process  analogous  to 
the  fertilization  of  each  gemmule  that  becomes  developed  into  a 
cell  by  means  of  the  partially  developed  cell  that  has  preceded  it  in 
the  regular  order  of  growth.     (I  look  on  it  as  due,  first,  to  the 
successive  segmentations  of  the  host  of  gemmules  that  are  con- 
tained in  the  newly  fertilized  ovum,  owing  to  their  mutual  affinities 
and  repulsions;  and,  secondly,  to  the  development  of  the  dominant 
or   representative    gemmules   in   each    segmentation,    the   others 
remaining  dormant,  and  are  called,  for  convenience,  in  the  next 
paragraph,  the  residue.) 

5.  "The  sexual  elements  are  formed  by  aggregation  out  of  the 
gemmules,  all  of  which  are  supposed  to  travel  freely  throughout 
the  body.     (I  look  on  the  sexual  elements  as  directly  descended 
from  the  'residue'  and   do  not  suppose  the  gemmules  to  travel 
freely.     I   allow   some   very   moderate   transgression   across   the 
bounds  of  their  domiciles,  and  something  more  than  that,  under 
the  limitations  that  will  be  described  in  the  latter  part  of  this 


CONFORMITY  TO  TYPE  257 

memoir.  I  account  for  all  varieties  of  the  gemmules  being  found 
in  all  parts  of  the  body  by  the  above-mentioned  segmentations 
being  never  clean  and  precise. 

"Hence  it  follows  that  each  segmentation  must  contain  stray 
and  alien  gemmules,  and  I  suppose  that  many  of  these  become 
entangled  and  find  lodgment  in  the  tissue  developed  out  of  each 
segmentation.) " 

Having  thus  enclosed  all  of  the  hereditary  matter  in 
the  germinal  cell  from  a  part  of  which  the  new  being 
develops,  and  the  "residue"  of  which,  by  multiplication 
of  its  units,  lays  the  foundation  for  the  next  generation, 
Galton  saw  that  it  would  be  inconsistent  that  variations 
should  occur  among  descendants.  He  says: 

"It  is  supposed  "that  the  structure  of  an  animal  changes  when 
he  is  placed  under  changed  conditions;  that  his  offspring  in- 
herit some  of  his  change;  and  that  they  vary  still  further  on 
their  own  account  and  in  the  same  direction,  and  so  on  through 
successive  generations  until  a  notable  change  in  the  congenital 
characteristics  of  the  race  has  been  effected.  Hence  it  is  concluded 
that  a  change  in  the  personal  structure  has  reacted  on  the  sexual 
elements."  "For  my  part  I  object  to  so  general  a  conclusion. 
.  .  .  We  might  almost  reserve  our  belief  that  the  structural 
cells  can  react  on  the  sexual  elements  at  all  and  we  may  be  con- 
fident that  they  do  so  in  a  very  fiant  degree;  in  other  words, 
that  acquired  modifications  are  barely,  if  at  all,  inherited,  in  the 
correct  sense  of  that  word."  "If  they  were  not  heritable,  then  a 
certain  group  of  cases  would  vanish,  and  we  should  be  absolved 
from  all  further  trouble  about  them;  but  if  they  exist,  in  however 
faint  a  degree,  a  complete  theory  of  heredity  must  account  for 
them."  "I  propose  ...  to  accept  the  supposition  of  their 
being  faintly  heritable  and  to  account  for  them  by  a  modification 
of  pangenesis."  "  Each  cell  may  be  supposed  to  throw  off  a  few 
germs  that  find  their  way  into  the  circulation,  and  thereby  to 
acquire  a  chance  of  occasionally  finding  their  way  to  the  sexual 
elements  and  of  becoming  naturalized  among  them." 

The  occasional  non-appearance  in  the  offspring  of 
qualities  for  which  the  parents  have  been  exceptionally 
remarkable,  and  which  may  reappear  in  the  third  genera- 
tion, is  attributed  by  Galton  to  the  particular  gemmules 
from  which  these  qualities  arise  having  become  tempora- 
rily exhausted  in  the  stirp,  in  which  they  again  appear 
by  multiplication  during  the  preparation  of  succeeding 
generations. 

The  doctrine  of  gemmules  formed  the  foundation 
of  another  theory  of  inheritance  suggested  by  Brooks. 
17 


258  BIOLOGY:  GENERAL  AND  MEDICAL 

Upon  superficial  examination  the  theory  closely  resem- 
bles the  theory  of  Darwin,  but  differs  from  it  in  certain 
important  points.  Thus,  though  Brooks  agrees  with 
Darwin  that  gemmules  are  given  off  by  all  the  cells  of 
the  body  and  that  they  circulate  in  the  blood  from 
which  they  concentrate  in  the  germ  cells,  he  differs  in 
ascribing  to  the  male  germ  cell  a  strong  affinity  or  attracting 
power  over  the  gemmules  so  that  it  collects  a  special 
mass  of  them  and  stores  them  up.  The  egg  cell  is  the 
conservative  principle  which  controls  the  transmission 
of  the  purely  racial  or  specific  characters,  whereas  the 
sperm  cell  is  the  progressive  element  which  causes  varia- 
tion. The  two  kinds  of  germ  cells  are  charged  with 
gemmules  in  different  degrees.  The  theory  is  chiefly 
aimed  at  the  explanation  of  variation  which  is  supposed 
to  be  caused  by  every  gemmule  of  the  spermatozoon 
uniting  with  that  particular  gemmule  of  the  ovum  that 
is  destined  to  give  rise,  in  the  offspring,  to  the  cell  which 
corresponds  to  the  one  which  produced  the  germ  or 
gemmule.  When  this  cell  becomes  developed  in  the 
body  of  the  offspring  it  will  be  a  hybrid  and  will  there- 
fore tend  to  vary. 

It  will  be  observed  by  the  thoughtful  reader  that  with 
the  progress  of  knowledge  more  attention  was  being 
devoted  to  the  germ  cells  and  less  to  the  somatic  cells. 
This  will  be  made  more  clear  by  the  thoughts  expressed 
in  the  ensuing  theories. 

Nageli  conceived  that  the  body  was  made  up  of  two 
different  materials,  the  trophoplasm  or  nutrient  plasm, 
and  idioplasm  or  germ  plasm.  The  latter,  though  present 
in  small  quantity,  determines  the  detailed  construction 
of  the  former.  He  conceived  the  idioplasm  to  form  a 
very  fine  network  of  fine  fibers  which  traverse  the  cells, 
continuing  from  cell  to  cell  so  that  all  parts  of  the  body 
become  pervaded  by  it  as  a  connected  network.  Proto- 
plasm, including  both  trophoplasm  and  idioplasm,  he 
considered  to  be  compounded  of  exceedingly  minute 
units  no  larger  than  a  molecule  of  albumen,  to  which  he 
gave  the  name  micellae.  These  were  capable  of  multi- 


CONFORMITY   TO   TYPE  259 

plication,  not  by  division,  but  through  the  formation  of 
new  ones  between  those  already  existing.  The  ger- 
minal cells  contain  both  trophoplasm  and  idioplasm,  the 
latter  governing  the  growth  of  the  former  as  it  increases 
itself.  In  this  theory  the  thought  of  the  continuity  of 
the  germinal  substance  is  foreshadowed.  Charles  Sedg- 
wick  Minot  suggested  that  Nageli's  "  idioplasm "  might 
be  identified  with  the  nuclear  chromatin. 

Gustav  Jager  in  1878  seems  to  have  been  the  first  to 
express  the  idea  that  in  the  higher,  organisms  the  body 
consists  of  two  kinds  of  cells  which  he  called  the  "  auto- 
genetic"  and  "  phylogenetic, "  respectively,  and  that  the 
latter  or  reproductive  cells  are  not  the  product  of  the 
former,  or  body  cells,  but  are  derived  directly  from  the 
germ  cell  of  the  parent. 

Rauber  in  1880  conceived  that  the  effect  of  fertilization 
was  to  convert  a  portion  of  the  egg,  namely,  the  personal 
part,  into  the  form  of  a  person;  the  other  portion  does 
not  experience  this  effect,  for  it  has  stronger  powers  of 
persistence. 

Nussbaum  in  1880  also  foreshadowed  the  idea  of  the 
continuity  of  the  germ  cells,  and  supposed  that  the  seg- 
mented ovum  divides  into  the  cell  material  of  the  indi- 
vidual and  the  cells  for  the  preservation  of  the  species. 
These  ideas  remained  unnoticed  until  Weismann's 
theory  was  evolved  in  1892. 

The  theory  of  the  "germ  plasm"  was  the  work  of 
Weismann  and,  though  it  may  be  subject  to  valid 
objections,  contains  so  large  a  proportion  of  truth  that 
it  has  taken  a  strong  hold  upon  the  thought  of  the  day 
and  forms  the  basis  of  a  large  part  of  biological  specula- 
tion. It  is  essentially  a  cytological  theory,  and  though 
it  follows  the  thought  expressed  in  the  theories  preceding 
it,  that  some  kind  of  physiological  units  are  engaged  in 
the  phenomena  of  heredity  and  centres  them  in  the  germ 
plasm  which  is. believed  to  be  continuous  from  genera- 
tion to  generation,  it  progresses  much  further  and  pro- 
poses to  show  the  source  of  the  germ  plasm  and  the  exact 
location,  distribution,  and  treatment  of  the  units. 


260  BIOLOGY:  GENERAL  AND  MEDICAL 

As  Weismann's  theory  is  of  such  importance,  it  will  be 
dwelt  upon  at  some  length  and  as  nearly  as  possible  be 
given  in  the  author's  own  language. 

"All  the  phenomena  of  heredity  depend  upon  minute  vital  units 
which  we  have  called  biophors  and  of  which  living  matter  is 
composed:  these  are  capable  of  assimilation,  growth,  and  multipli- 
cation by  division.  We  are  unacquainted  with  the  lowest  con- 
ceivable organisms,  and  do  not  even  know  if  they  still  exist. 
But  they  must  at  any  rate  have  done  so  at  some  time  or  other, 
in  the  form  of  single  biophors,  in  which  multiplication  and  trans- 
mission occurred  together,  no  special  mechanism  for  the  purpose 
of  heredity  being  present.  When  the  body  became  constructed  in 
a  more  or  less  complex  manner,  of  various  kinds  of  biophors 
arranged  in  a  definite  manner,  simple  binary  fission  no  longer 
sufficed  for  the  transmission  of  the  characters  of  the  parent  to  the 
offspring.  If  the  parts  situated  in  the  anterior,  posterior,  right, 
left,  dorsal,  and  ventral  regions  differed  from  one  another,  all  the 
elements — i.e.,  all  the  kinds  and  groups  of  biophors— could  not 
by  any  method  of  halving,  be  transmitted  to  both  the  offspring 
resembling  the  parent.  Special  means  must  then  have  been 
adopted  to  render  such  a  completion  and  consequent  perfect 
transmission  possible;  and  this  was  attained  by  the  formation  of  a 
nucleus.  We  may  regard  the  nucleus  as  having  originally  served 
merely  for  the  storage  of  reserve  biophors.  Subsequently — that 
is,  in  multicellular  organs  possessing  highly  differentiated  cells — 
the  nucleus  took  on  other  functions,  which  regulated  the  specific 
activity  of  the  cell,  though  it  still  retained  biophors  capable  of 
supplying  the  characters  of  the  cells  which  were  still  wanting  and 
therefore  still  better  served  as  the  bearer  of  the  biophors  con- 
trolling the  character  of  the  cell.  If,  therefore,  a  special  apparatus 
for  transmission  became  necessary  in  the  hetero-biophorids  or 
unicellular  organisms  and  appeared  in  the  'cell '  in  the  form  of  a 
'nucleus,'  it  must  have  become  still  more  complex  on  the  intro- 
duction of  the  remarkable  process  of  amphimixis,  which,  in  its 
simplest  and  original  form,  consists  in  the  complete  fusion  of  two 
organisms  in  such  manner  that  nucleus  unites  with  nucleus  and 
cell  body  with  cell  body  (conjugation).  In  the  higher  unicellular 
organisms  this  process  is,  in  most  cases,  restricted  to  the  fusion  of 
the  nuclei  half  the  nucleus  of  one  animal  uniting  with  half  that  of 
another.  The  process  of  division  shows  that  the  nucleus  has  a 
structure  precisely  analogous  to  that  of  the  nucleus  in  multicellular 
organisms;  we  may,  therefore,  assume  that  the  hereditary  sub- 
stance here  likewise  consists  of  several  equivalent  groups  of 
biophors,  constituting,  'nuclear  rods'  or  'idants,'  each  of  which 
contains  all  the  kinds  of  biophors  of  the  organism,  though  they 
deviate  slightly  from  one  another  in  their  composition  as  they 
correspond  to  individual  variations.  Half  the  idants  of  two 
individuals  become  united  in  the  process  of  amphimixis  and  thus  a 
fresh  intermixture  of  individual  characters  results. 

"The  apparatus  for  transmission  in  those  multicellular  organ- 
isms in  which  the  cells  have  undergone  a  division  of  labor  is  essen- 
tially similar  to  that  seen  in  unicellular  beings;  although  in 


CONFORMITY   TO   TYPE  261 

correspondence  with  the  greater  complexity  of  their  structure  it  is 
more  complicated. 

"As  the  process  of  amphimixis  occurs  in  them  also,  and  the  fission 
of  the  highly  differentiated  multicellular  individuals  seems  to  be 
only  possible  by  a  temporary  return  to  the  unicellular  condition, 
we  find  that  the  so-called  'sexual  reproduction/  which  is  of 
general  occurrence  among  them,  consists  in  all  the  primary  con- 
stituents (Anlagen)  of  the  entire  organism  being  collected  together 
in  the  nucleus  matter  of  a  single  reproductive  cell. 

"Two  kinds  of  such  cells,  which  are  differently  equipped  and 
mutually  attract  one  another,  then  unite  in  the  process  of  amphi- 
mixis and  constitute  what  we  are  accustomed  to  call  the  'fertilized 
egg  cell,'  which  contains  the  combined  hereditary  substance  of 
two  individuals.  According  to  our  view,  this  hereditary  substance 
of  the  multicellular  organisms  consists  of  three  orders  of  vital 
units,  the  lowest  of  which  is  constituted  by  the  biophors.  In  the 
unicellular  forms  a  more  or  less  polymorphic  mass  of  biophors 
having  a  definite  arrangement,  constitutes  the  individual  nuclear 
rods  or  idants  (chromosomes),  several  of  these  making  up  the 
hereditary  substance  of  the  nucleus  which  controls  the  cells;  and 
similarly  in  these  higher  forms,  groups  of  biophors,  arranged  in  a 
certain  order,  constituting  the  primary  constituents  of  .the  in- 
dividual cells  of  the  body,  and  together  form  the  second  order  of 
vital  units — the  determinants.  The  histological  character  of  every 
cell  in  a  multicellular  organism,  including  its  rate  and  mode  of 
division,  is  controlled  by  such  a  determinant.  The  germ  cell, 
however,  does  not  contain  a  special  determinant  for  every  cell 
unless  it  is  to  remain  independently  variable.  The  germ  cell  of  a 
species  must  contain  as  many  determinants  as  the  organism  has  cells 
or  groups  of  cells  which  are  independently  variable  from  the  germ 
onwards,  and  these  determinants  must  'have  a  definite  mutual 
arrangement  in  the  germ  plasm,  and  must  therefore  constitute  a 
definitely  limited  aggregate,  or  higher  vital  unit,  the  'id.'  From 
the  facts  of  sexual  reproduction  and  heredity  we  must  conclude 
that  the  germ  plasm  contains  many  ids,  and  hot  a  single  one  only. 
The  formation  of  hybrids  proves  that  the  two  parents  together 
transmit  all  their  specific  characters,  so  that  in  the  process  of 
fertilization  each  contributes  a  hereditary  substance  which  con- 
tains the  primary  constituents  of  all  parts  of  the  organism — that  is, 
all  the  determinants  required  for  building  up  the  new  individual. 
The  hereditary  substance  becomes  halved  at  the  final  stage  of 
development  of  the  germ  cells,  and  consequently  all  the  deter- 
minants must  previously  have  been  grouped  into  at  least  two  ids. 
But  it  is  very  probable  that  many  more  ids  are  usually  present 
and  that  in  many  cases  their  number  far  exceeds  a  hundred.  It 
cannot  be  stated  with  certainty  which  portions  of  the  elements 
of  the  germ  plasm  observable  in  the  nucleus  of  the  ovum  correspond 
to  the  ids,  though  it  is  probable  that  only  parts  of  and  not  the 
entire  'chromosomes'  are  to  be  regarded  as  such.  Until  this 
point  can  be  definitely  decided,  our  further  detailed  deductions 
will  be  based  on  the  view  that  the  nuclear  rods  (chromosomes) 
are  aggregates  of  ids,  which  we  speak  of  as  '  idants.'  In  a  certain 
sense  these  are  also  vital  units  for  they  grow  and  multiply  by 


262  BIOLOGY:  GENERAL  AND  MEDICAL 

division;  and  the  combination  of  ids  contained  in  them,  although 
not  a  permanent  one,  persists  for  some  time. 

"The  '.germ  plasm,  or  hereditary  substance  of  the  metazoa  and 
metaphyta,  therefore,  consists  of  a  larger  or  smaller  number  of 
idants,  which  in  turn  are  composed  of  ids;  each  id  has  a  definite 
and  special  architecture,  as  it  is  composed  of  determinants,  each 
of  which  plays  a  perfectly  definite  part  in  development. 

"The  development  of  the  primary  constituents  in  the  germ 
plasm  of  the  reproductive  cell  takes  place  in  the  course  of  the  cell 
divisions  to  which  the  autogeny  of  a  multicellular  organism  is  due, 
in  which  process  all  the  ids  behave  in  exactly  similar  manner. 
In  the  first  cell-division  every  id  divides  into  two  halves,  each  of 
which  contains  only  half  the  entire  number  of  determinants; 
and  this  process  of  disintegration  is  repeated  at  every  subsequent 
cell-division,  so  that  the  ids  of  the  following  autogenetic  stages 
gradually  become  poorer  as  regards  the  diversity  of  their  determin- 
ants, until  they  finally  contain  only  a  single  kind. 

"Each  cell  in  every  stage  is  in  all  cases  controlled  by  only  one 
kind  of  determinant,  but  several  of  the  same  kind  may  be  con- 
tained in  the  id;  and  the  'control'  of  the  cell  is  effected  by  the 
disintegration  of  the  determinants  into  biophors  which  penetrate 
through  the  nuclear  membrane  into  the  cell  body;  and  there, 
according  to  definite  laws  and  forces  of  which  we  are  ignorant, 
bring  about  the  histological  differentiation  of  the  cell,  by  multiply- 
ing more  rapidly  at  the  expense  of  those  biophors  already  forming 
in  the  cell  body.  Each  determinant  must,  become  'ripe/  and 
undergo  disintegration  into  its  biophors  at  a  definite  time  or  at  a 
certain  stage  of  autogeny.  The  rest  of  the  determinants  in  the  id 
of  a  cell,  which  are  destined  for  subsequent  stages,  remain  intact, 
and  have  therefore  no  effect  on  the  control  of  the  cell;  but  the 
mode  of  their  arrangement  in  the  id  and  the  special  rate  of 
multiplication  of  each  kind  determine  the  nature  of  the  next 
nuclear  division — that  is,  as  to  which  determinants  are  to  be 
distributed  to  one  daughter  cell  and  which  to  the  other.  The 
histological  nature  of  these  two  cells,  as  well  as  the  control  of  their 
successors,  is  determined  by  this  division;  and  thus  the  distribution 
of  the  primary  constituents  contained  in  the  germ  plasm  is 
effected  by  the  architecture  of  the  id,  which  is  at  first  a  definite 
kind,  but  afterward  undergoes  continual  and  systematic  changes 
in  consequence  of  the  uneven  rate  of  multiplication  and  gradual 
disintegration  of  the  ids. 

"  The  apparatus  for  cell-division  is  only  of  secondary  importance 
in  the  process;  its  chief  part,  the  centrosome,  like  the  hereditary 
substance,  is  derived  from  the  parental  germ  cell  or  cells,  but  only 
constitutes  the  mechanism  for  the  division  of  the  nucleus  and  cell 
and  contains  no  'primary  constituents.'  The  rate  of  cell-divisions 
cannot,  moreover,  be  determined  by  the  centrosome,  although  it 
produces  the  primary  stimulus;  the  apparatus  for  division  is  set 
in  motion  by  the  cell,  which  is  controlled  by  the  idioplasm.  Were 
this  not  the  case,  the  nuclear  matter  could  not  be  the  hereditary 
substance,  for  most  of  the  hereditary  characters  of  a  species  are  due 
in  a  less  degree  to  the  differentiation  of  individual  cells  than  to  the 
number  and  grouping  of  the  cells  of  which  a  certain  organ  or 


CONFORMITY   TO    TYPE  263 

entire  part  of  the  body  consists;  these,  however,  again  depend  on 
the  mode  and  rate  of  cell-division. 

"The  processes  occurring  in  the  idioplasm  which  direct  the 
development  of  the  organism  from  the  ovum — or,  to  speak  in  more 
general  terms,  from  one  cell,  the  germ  cell — do  not  in  themselves 
furnish  an  explanation  of  a  series  of  phenomena  which  are  in  part 
directly  connected  with  the  autogeny,  or  else  result  from  it  sooner 
or  later;  the  phenomena  of  regeneration,  gemmation,  and  fission, 
and  the  formation  of  new  germ  cells,  all  require  special  supple- 
mentary hypotheses. 

"The  simplest  cases  of  regeneration  are  due  to  the  fully  formed 
tissue,  consisting  of  similar  cells,  always  containing  a  reserve  of 
young  cells,  which  are  capable  of  replacing  a  normal  or  abnormal 
loss.  This  is,  however,  insufficient  in  the  more  complex  cases, 
in  which  entire  parts  of  the  body,  such  as  the  tail  or  limbs,  are 
regenerated  when  they  have  been  forcibly  removed.  We  must 
here  assume  that  the  cells  of  the  parts  which  are  capable  of 
regeneration  contain  'supplementary  determinants'  in  addition 
to  those  which  control  them,  and  that  these  are  the  primary 
constituents  of  the  parts  which  are  formed  anew  in  the  process 
of  regeneration.  They  are  supplied  to  certain  parts  of  the  body 
at  an  earlier  autogenetic  stage  in  the  form  of  'inactive  accessory 
idioplasm,'  and  only  become  active  when  the  opposition  to  growth 
has  been  removed  in  consequence  of  the  loss  of  the  part  in  question. 
The  equipment  of  a  cell  of  any  part  with  supplementary  determin- 
ants presupposes  a  greater  complexity  in  their  distribution,  in 
correspondence  with  the  greater  complexity  in  structure  of  the 
part;  and  thus  the  capacity  for  regeneration  is  limited,  for  a  part 
can  no  longer  be  provided  with  an  apparatus  for  regeneration 
when  its  structure  is  too  complicated.  The  ordinary  assumption 
that  the  regenerative  'force'  decreases  as  the  complexity  in 
structure  increases  is  therefore  to  a  certain  extent  true,  but  not 
if  it  implies  the  existence  of  a  special  force  which  provides  for 
regeneration  and  which  always  diminishes  in  correspondence 
with  the  degree  of  organization. 

"Reproduction  by  fission  is  closely  connected  with  regeneration; 
it  presupposes  the  existence  of  a  similar  apparatus  in  the  idioplasm, 
which,  however,  has  in  most  cases  reached  a  higher  stage  of 
development:  fission  must  have  arisen  phyletically  from  regener- 
ation. 

"The  origin  of  multiplication  by  gemmation  and  the  phenomena 
exhibited  by  this  form  of  reproduction  are  different  from  those 
concerned  in  fission.  In  plants  and  Coelenterates  gemmation 
originates  in  one  cell,  which  must  consequently  contain  a  com- 
bination of  all  the  determinants  of  the  species  closely  resembling 
that  existing  in  the  fertilized  ovum.  In  the  Polyzoa,  however, 
this  process  does  not  originate  in  one  cell,  but  in  at  least  two,  and 
probably  more,  belonging  to  two  different  layers  of  cells  (germinal 
layers)  of  the  body;  and  in  Tunicata,  again,  the  material  for  the 
bud  is  produced  from  all  three  germinal  layers.  The  first  of  these 
forms  of  budding  must  be  primarily  due  to  the  mixture  of  '  unalter- 
able '  germ  plasm  to  certain  series  of  cells  in  autogeny  in  the  form 
of  inactive  '  accessory  idioplasm,'  or  '  blastogenic '  idioplasm.  In 
plants  this  is  contained  in  the  apical  cells,  and  in  the  hydroid 


264  BIOLOGY:  GENERAL  AND  MEDICAL 

polyps  in  the  cells  of  the  ectoderm.  In  the  second  group  of 
animals  mentioned,  we  must  assume  that  the  '  blastogenic '  germ- 
plasm  becomes  disintegrated  into  two  groups  of  determinants  at 
an  early  autogenetic  stage  and  that  each  of  these  is  passed  on  in  an 
'unalterable'  condition,  through  various  generations  of  cells,  until 
the  time  and  place  of  its  activity  are  reached.  In  the  third  group, 
the  inactive  '  blastogenic '  idioplasm  divides  into  three  groups  of 
determinants,  one  of  which  passes  into  the  ectoderm,  the  second 
into  certain  cell  series  of  the  mesoderm,  and  the  third  into  others 
in  the  endoderm,  until  they  reach  the  part  in  which  they  have 
to  become  active. 

"  Gemmation  must  have  originated  phyletically  by  a  doubling  of 
the  germ  plasm  taking  place  in  the  fertilized  egg  so  that  one 
half  remained  inactive  and  was  then  passed  on  as  inactive  '  blasto- 
genic' germ  plasm,  or  else  became  divided  up  in  the  course  of 
autogeny  into  groups,  which  were  passed  separately  to  the  same 
region,  viz.,  that  of  the  bud. 

"We  assume  that  two  kinds  of  germ  plasm  exist  in  those  species 
in  which  alternation  of  generation  occurs,  both  of  which  are  present 
in  the  egg  cell  as  well  as  in  the  bud,  though  only  one  of  them  is 
active  at  a  time  and  controls  autogeny  while  the  other  remains 
inactive.  The  alternating  activity  of  these  two  germ  plasms 
causes  the  alternation  of  generations. 

" The  formation  of  germ  cells  is  brought  about  by  the  occurrence 
of  similar  processes  in  the  idioplasm  to  those  which  cause  gem- 
mation. One  part  of  the  germ-plasm  contained  in  the  fertilized 
egg  cell  remains  inactive  and  '  unalterable' ;  that  is,  it  does  not 
immediately  become  disintegrated  into  groups,  but  is  passed  on  in 
the  form  of  accessory  idioplasm  to  certain  series  of  cells  in  autogeny, 
and  thus  reaches  the  parts  in  which  germ  cells  are  to  be  formed. 
Thus,  the  whole  of  the  parental  germ  plasm,  with  all  its  determin- 
ants, forms  the  foundation  of  the  germ  cells  which  give  rise  to  the 
next  generation,  and  the  extremely  accurate  and  detailed  trans- 
mission of  the  parental  characters  to  the  offspring  is  thereby 
rendered  comprehensive. 

"In  multicellular  plants  and  animals,  the  germ  plasm  becomes 
more  complex  in  consequence  of  sexual  reproduction,  in  which 
process  the  ids  of  two  different  individuals,  the  parents,  are 
accumulated  in  the  fertilized  egg  cell  every  time  amphimixis 
occurs.  This  has  caused  the  occurrence  of  the  '  reducing  division/ 
which  accompanies  the  formation  of  male  and  female  germ  cells, 
and  results  in  the  number  of  ids  and  idants  being  reduced  to  the 
half.  As  the  reduction  does  not  always  occur  in  the  same  way. 
and  the  resulting  halves  contain  different  idants  on  different 
occasions,  and  these  fall  to  the  share  of  individual  germ  cells,  it  is 
possible  for  the  germ  cells  of  one  individual  to  contain  very 
different  combinations  of  idants.  This  results  in  the  dissimilarity 
between  the  offspring  of  the  same  parents  or,  to  express  it  in  more 
general  terms,  in  the  extreme  diversity  as  regards  the  intermixture 
of  individual  differences. 

"The  type  of  the  child  is  determined  by  the  paternal  and 
maternal  ids  contained  in  the  corresponding  germ  cells  meeting 
together  in  the  process  of  fertilization,  and  the  blending  of  the 
parental  and  ancestral  characters  is  thus  predetermined,  and 


CONFORMITY   TO   TYPE  265 

cannot  become  essentially  modified  by  subsequent  influences. 
The  facts  relating  to  identical  twins  and  to  plant  hybrids  prove 
that  this  is  so. 

"Reversion  to  grandparents  and  great-grandparents  or  to 
uncles  and  aunts  may  be  accounted  for  by  the  fact  that,  in  the 
first  place,  the  idants  and  the  ids  are  not  formed  anew  in  the  germ 
plasm  of  the  parents,  but  are  derived  from  the  grandparents;  and, 
secondly,  that  the  combination  of  ids  contained  in  the  individual 
germ  cells  of  the  parents  become  very  diversified  in  consequence 
of  the  'reducing  division.' 

"The  number  of  ids  of  any  particular  ancestor  which  are  con- 
tained in  the  germ  plasm  of  a  ripe  germ  cell  depends  entirely  on  the 
manner  in  which  the  reducing  division  occurs;  and,  under  certain 
circumstances,  a  germ  cell  might  presumably  contain  half  the 
entire  number  of  ids  of  one  grandparent  and  none  of  those  of  the 
other  three.  The  larger  the  number  of  ids  derived  from  an 
ancestor,  the  greater  is  the  probability  that  some  of  the  characters 
of  this  ancestor  will  appear  in  the  descendants;  but  this  depends 
on  the  force  of  the  ids  of  the  other  parent  which  comes  into  play 
when  amphimixis  takes  place,  and  also  on  whether  the  ids  derived 
from  this  ancestor  are  the  dominant  ones  which  determine  his 
'  type.' 

"From  this  theory  it  could  be  predicted  that  hybrid  plants 
fertilized  with  their  own  pollen  must  produce  very  variable  off- 
spring, and  that  individuals  of  these  hybrids  must,  moreover, 
revert  to  one  or  other  of  the  ancestral  species;  both  these  state- 
ments are  borne  out  by  fact. 

"The  more  remote  the  ancestors  to  the  characters  of  which 
reversion  occurs,  the  more  rarely  will  it  take  place.  Reversion  to 
the  three-toed  ancestors  of  the  horse,  for  instance,  is  of  extremely 
rare  occurrence  for  it  is  due  to  the  retention  of  ancestral  determin- 
ants which  have  certainly  disappeared  from  all  the  ids  in  the  germ 
plasm  of  most  existing  horses. 

"The  remarkable  phenomenon  of  dimorphism,  which  has  been  so 
extensively  introduced — more  especially  into  the  animal  kingdom 
— by  means  of  sexual  reproduction  must  be  due  to  the  presence 
in  the  idioplasm  of  double  determinants  for  all  those  cells,  groups 
of  cells  and  entire  organisms  which  are  capable  of  taking  on  a  male 
and  female  form.  But  only  one  half  of  such  a  double  determinant 
remains  inactive,  while  the  other  remains  active.  The  sexual 
differentiation  of  the  germ  cells  must  thus  be  due  to  the  presence 
of  Spermatogenetic  and  Oogenetic  double  determinants;  and  even 
all  the  secondary  sexual  characters  must  be  traced  to  a  similar 
origin  in  the  idioplasm. 

"The  assumption  of  double  determinants  is  also  able  to  throw 
some  light  upon  certain  enigmatical  phenomena  of  heredity 
exhibited  by  human  beings.  It  has  long  been  known  that  hemo- 
philia (the  bleeder's  disease)  occurs  in  men  only,  but  is  trans- 
mitted by  women.  This  disease,  like  a  secondary  sexual  character, 
is  only  transmitted  to  the  sex  in  which  it  first  appeared,  for  this 
half  of  the  double  determinants  of  the  'mesoblast  germ'  has  alone  been 
modified  by  the  disease. 

"  It  is  self-evident  from  the  theory  of  heredity  here  propounded 
that  only  those  characters  are  transmissible  which  have  been 


266  BIOLOGY:  GENERAL  AND  MEDICAL 

controlled — i.e.,  produced — by  determinants  of  the  germ,  and  that 
consequently  only  those  variations  are  hereditary  which  result 
from  the  modification  of  several  or  many  determinants  in  the 
germ  plasm,  and  not  those  which  have  arisen  subsequently  in  con- 
sequence of  some  influence  exerted  upon  the  cells  of  the  body.  In 
other  words,  it  follows  from  this  theory  that  somatogenic  or 
acquired  characters  cannot  be  transmitted. 

"This,  however,  does  not  imply  that  external  influences  are 
incapable  of  producing  hereditary  variations;  on  the  contrary,  they 
always  give  rise  to  such  variations  when  they  are  capable  of 
modifying  the  determinants  of  the  germ  plasm.  Climatic  in- 
fluences, for  example,  may  well  produce  permanent  variations  by 
slowly  causing  gradually  increasing  alterations  to  occur  in  the 
determinants  in  the  course  of  generations.  The  primary  cause  of 
variation  is  always  the  effect  of  external  influences.  When 
deviations  only  affect  the  soma,  they  give  rise  to  temporary, 
non-hereditary  variations;  but  when  they  occur  in  the  germ  plasm, 
they  are  transmitted  to  the  next  generation  and  cause  correspond- 
ing hereditary  variations  in  the  body." 

The  chief  feature  of  Weismann's  theory  is  thus  ex- 
pressed by  Wilson:  "It  is  a  reversal  of  the  true  point  of 
view  to  regard  inheritance  as  taking  place  from  the  body 
of  the  parent  to  that  of  the  child.  The  child  inherits 
from  the  parent  germ  cell,  not  from  the  parent  body, 
and  the  germ  cell  owes  its  characteristics  not  to  the  body 
which  bears  it,  but  to  its  descent  from  a  pre-existing 
germ  cell  of  the  same  kind.  Thus  the  body  is,  as  it  were, 
an  offshoot  from  the  germ  cell.  As  far  as  inheritance 
is  concerned,  the  body  is  merely  the  carrier  of  the  germ 
cells  which  are  held  in  trust  for  coming  generations." 

It  goes  without  saying  that  so  suggestive  and  com- 
plete a  theory  as  that  of  Weismann  must  take  a  strong 
hold  and  leave  a  deep  impression  upon  the  thoughtful 
mind.  Whatever  may  be  thought  of  the  biophors, 
determinants,  ids,  and  idants — and  some,  like  Adami, 
believe  that  they  have  demolished  this  elaborate  succes- 
sion by  showing  the  physical  impossibility  of  a  sufficient 
number  of  them  being  packed  away  in  the  germ  plasm — 
the  doctrine  of  the  continuity  of  the  germ  plasm  re- 
mains unassailed  and  forms  the  foundation  of  much  of 
the  thought  of  the  present  day. 

During  these  lengthy  excerpts  from  the  writings  upon 
inheritance  the  reader  cannot  but  have  observed  that 


CONFORMITY   TO   TYPE  267 

an  important  difficulty  to  be  overcome,  and  one  upon 
which  considerable  time  has  been  spent,  is  the  appearance 
in  the  offspring  of  peculiarities  not  found  in  his  parents, 
though  present  in  his  earlier  forbears.  Light  upon  this 
obscure  subject  is  found  in  the  thoughtful  and  important 
work  of  -Gregor  Johann  Mendel,  an  Austrian  botanist, 
who  for  a  number  of  years  studied  the  phenomena  of 
hybridity  among  certain  peas.  His  writings,  being 
published  in  an  obscure  journal,  were  overlooked  partly 
for  that  reason  and  partly  because  they  appeared  in 
1866  when  Darwin  was  impressing  the  whole  world 
with  his  plausible  theory  of  the  "Origin  of  Species  by 
Natural  Selection"  which  so  changed  scientific  thought 
as  to  make  experiments  upon  hybridity  appear  futile. 


Line  of  succession  of 
individuals. 


-y   Line  of  here- 
ditary trans- 
mission. 

FIG.  100.— Heredity  of  germ  cells  and  somatic  cells.    G,  Germ  cells;   S,  somatic 
cells.    (Lock.) 

The  writing  was  discovered  in  1900  by  de  Vries,  who 
called  attention  to  its  great  importance,  excited  interest 
in  Mendel's  problems,  and  aroused  such  enthusiasm 
among  scientists  generally  that  the  paper  was  translated 
and  republished  with  an  introductory  note  by  W. 
Bateson  in  the  Journal  of  the  Royal  Horticultural  Society 
of  London,  Vol.  XXIV,  1901-02. 

In  considering  Mendel's  work  it  must  not  be  forgotten 
that  it  is  a  study  of  hybrid  characters  and  that  in  conse- 
quence all  that  was  found  need  not  apply  to  normal  repro- 
duction. But  on  the  other  hand,  the  study  of  hybrids, 
the  striking  dissimilarities  of  whose  parents  are  combined 
in  the  offspring,  enables  us  to  trace  given  characters 
with  ease  because  of  their  conspicuousness.  It  is  the 


268  BIOLOGY:  GENERAL  AND  MEDICAL 

ability  to  recognize  the  Mendelian  characters  that  has 
made  it  possible  to  formulate  a  law  as  to  their  mode  of 
transmission. 

Mendel  worked  with  peas  because  the  sexual  organs  of 
these  flowers  are  enclosed  by  the  petals  in  such  manner 
that  self-fertilization  is  inevitable.  To  make  the  hybrids, 
he  had  to  cut  away  part  of  the  flower,  remove  the  unripe 
anthers,  and  at  the  time  of  the  maturation  of  the  stigma 
apply  such  pollen  as  was  desired. 

It  was  soon  discovered  that  certain  characters  of 
the  peas  were  traceable  from  generation  to  generation, 
appearing  in  recognizable  form  in  the  normal  individual 
and  in  the  hybrid.  Such  characters  are,  for  example, 
color  of  the  flower,  size  of  the  plant,  quantity  of  sugar 
in  the  seed,  and  quantity  of  starch  in  the  seed.  These 
characters  which  appear  to  blend  with  their  opposites  in 
the  hybrids  of  the  first  generation  are  found  by  an 
examination  of  the  second  generation  to  have  effected 
a  temporary  combination  which  loosens  up  and  begins 
to  separate,  so  that  with  each  succeeding  generation  a 
greater  number  of  the  offspring  revert  to  the  parental 
types  until  after,  say,  ten  generations  scarcely  any  hybrid 
organisms  remain. 

These  facts  were  in  thorough  accord  with  well-known 
facts  concerning  flowers.  Many  of  our  most  beautiful 
garden  flowers  are  hybrids,  some  of  which  were  produced 
only  after  infinite  pains  had  been  taken  in  their  cultiva- 
tion. If  they  are  fertile  and  "go  to  seed,"  everyone 
that  has  enjoyed  a  garden  knows  with  what  dismay 
he  views  the  plants  growing  from  the  hybrid  seeds  which 
yield  a  few  of  the  desired  forms  among  a  large  number 
of  simple  and  commonplace  flowers.  Though  this  fact 
was  known  in  Mendel's  time,  and  it  was  generally  con- 
ceded that  hybrids  "tended  to  revert  to  the  primitive 
types,"  he  alone  had  the  genius  to  follow  the  matter  with 
scientific  accuracy,  to  reduce  the  reversion  to  a  mathe- 
matical basis,  and  to  lay  the  foundation  of  a  new 
principle  of  much  importance  in  studying  the  problems 
of  heredity. 


CONFORMITY   TO   TYPE 


269 


The  disposition  of  these  characters  traceable  from 
either  parent  into  the  hybrid  shows  that  each  hybrid 
is  not,  so  to  speak,  composited  of  two  factors,  but 
compounded  of  many  units,  which  must  occur  as  such 
in  the  germ  plasm.  To  such  of  these  units  as  are 
found  to  undergo  segregation  in  the  germ  plasm  of  the 


r  RC 

•2  RC 

=i    BHD  mac  IHIO 

=1      1  IR   1  IR  HMD 

Qatto 

IRR    :          21 

IR      : 

3R          :     IDD 

3D 

sr  —  i 

XCZ]R 

DHH^HHD 

RC=I  |  BHBO 

r  

DMBH 
RCZ3 

»  j  •  i  r 

-*  —  » 

1  IR 

•       1  D 

OHH    HH3 

RC=3     HMD 

77  n 

FIG.  101. — Schema  of  Mendel's  law  for  a  single  pair  of  "antagonistic"  proper* 
ties:  A,  The  results  of  hybridization  of  a  pure  dominant  (D)  with  a  pure 
recessive  (R)  form;  B,  the  results  of  crossing  a  hybrid  with  a  recessive  form 
(50  per  cent,  of  progeny  pure  recessive,  50  per  cent,  hybrid  but  apparently 
dominant) ;  C,  the  result  of  crossing  a  hybrid  with  a  dominant  form,  all  apparenty 
dominant  (but  50  per  cent,  pure,  50  hybrid).  (Bateson.)  (P)  signifies  the  parental 
combination;  FI  and  F2,  the  first  and  second  filial  combinations. 

hybrid,  and  consequent  separation  in  its  offspring,  Bateson 
has  given  the  name  Allelomorphs.  The  allelomorphic 
units  are  found  to  go  in  pairs,  the  individuals  being 
separable  and  therefore  capable  of  varying  combinations. 
Germinal  cells  in  which  the  allelomorphic  units  are  of 
the  same  kind  are  said  to  be  homozygous;  those  in  which 
they  are  of  opposed  kinds,  heterozygous. 


270  BIOLOGY:  GENERAL  AND  MEDICAL 

The  accompanying  diagram  will  assist  the  reader  in 
forming  a  concept  of  these  allelomorphs,  their  occur- 
rence in  pairs,  their  possible  separation,  and  their  appear- 
ance in  new  combinations. 

It  is  difficult  to  find  a  satisfactory  concise  definition  of 
what  is  known  as  Mendel's  law.  Indeed,  he  did  not 
formulate  it  in  words  himself. 

The  facts  upon  which  the  law  is  based  are  displayed 
in  the  following  tabulation  taken  from  Mendel's  own 
paper,  and  show  that  interbreeding  among  hybrids 
results  in  the  progressive  separation  of  the  combined 
characters  and  in  increasing  number  of  reversions  to  the 
pure  specific  types.  Before  the  tabulation  can  be  under- 
stood, however,  it  becomes  necessary  to  say  a  few  words 
about  the  terms  dominant  and  recessive  as  applied  to 
the  Mendelian  characters.  The  characters,  which  it 
will  be  remembered  go  in  pairs,  are  opposed  to  one  another 
in  quality  and  take  precedence  over  one  another  in 
character.  Thus,  whiteness  and  blackness  are  opposed 
Mendelian  characters,  of  which  blackness  takes  prece- 
dence over  the  whiteness,  and  is,  therefore,  dominant;  i.e., 
whenever  blackness  is  present  it  can  be  seen  whether 
there  be  some  whiteness  in  the  individual-  or  not,  but 
whiteness  may  be  present  and  completely  concealed  by 
blackness.  Mendel's  tabulation  of  what  happens  when 
only  two  sets  of  characters  are  concerned  is  shown  thus: 

A.  Dominant  character;  a,  recessive  character;  Aa, 
hybrid.  For  brevity's  sake  it  is  assumed  that  each 
plant  produces  only  four  seeds  (peas). 

Generation      A          Aa        a  Ratio 

Aa :   a 


1  1211 

2  6463 

3  28  8         28  7 

4  120         16       120         15 

5  496         32       496         31 


2  :    1 

2  :   3 

2  :   7 

2  :15 

2  :31 


2—1:2"  :2— 1 


CONFORMITY   TO   TYPE  271 

In  the  tenth  generation,  therefore,  2n— 1  =  1023.  In 
each  2,048  plants  that  arise  in  this  generation  1,023  will 
show  constant  dominant  character,  1,023  will  show 
constant  recessive  character,  and  only  two  will  be  hybrids. 

Thus,  there  is  a  spontaneous  tendency  to  escape  from 
hybridity,  taking  place  under  the  most  favorable  condi- 
tions with  a  regularity  susceptible  of  mathematical 
calculation  and  of  formulation  as  a  law. 

Spillman  expresses  the  Mendelian  law  thus:  "In  the 
second  and  later  generations  of  a  hybrid,  every  possible 
combination  of  the  parent  character  occurs,  and  each 
combination  appears  in  a  definite  proportion  of  the  indi- 
viduals." With  more  technical  diction  Lock  defines  it 
thus:  "The  gametes  of  a  heterozygote  bear  the  pure 
parental  allelomorphs  completely  separated  from  one 
another,  and  the  numerical  distribution  of  the  separate 
allelomorphs  in  the  gametes  is  such  that  all  possible 
combinations  of  them  are  present  in  approximately 
equal  numbers.  "  .  ...  "The  male  and  female  germ  cells 
of  hybrid  plants  contain  each  of  them  one  or  the  other 
member  only  of  any  pair  of  differentiating  characters 
exhibited  by  the  parents,  and  each  member  of  such  a 
pair  of  characters  is  represented  in  an  equal  number  of 
germ  cells  of  both  sexes.  Separate  pairs  of  differen- 
tiating characters  (allelomorphs)  conform  to  this  law  in 
complete  independence  of  one  another. 

DeVries  expresses  Mendel's  law  in  the  following  simple 
language:  "The  pairs  of  antagonistic  characters  in  the 
hybrid  split  up  in  their  progeny,  some  individuals  re- 
verting to  the  pure  parental  types,  some  crossing  with 
each  other  anew,  and  so  giving  rise  to  new  generations  of 
hybrids.  ...  In  fertilization  the  characters  of  both 
parents  are  not  uniformly  mixed,  but  remain  separated, 
though  most  intimately  combined  in  the  hybrid  through 
out  life.  They  are  so  combined  as  to  work  together 
nearly  always,  and  to  have  nearly  equal  influence  on  all 
the  processes  of  the  whole  individual  evolution.  But 
when  the  time  arrives  to  produce  progeny,  or  rather  to  pro- 


272  BIOLOGY:  GENERAL  AND  MEDICAL 

duce  the  sexual  cells  through  the  combination  of  which 
the  offspring  arises,  the  two  parental  characters  leave  each 
other  and  enter  separately  into  the  sexual  cells.  From 
this  it  may  be  seen  that  one-half  of  the  pollen  cells  will 
have  the  quality  of  one  parent  and  the  other  the  quality 
of  the  other.  And  the  same  holds  good  for  the  egg  cells. 
Obviously,  the  qualities  lie  latent  in  the  pollen  and  in  the 
egg,  but  ready  to  be  evolved  after  fertilization  has  taken 
place." 

According  to  Mendel's  law,  the  unexpected  appearance 
in  the  offspring  of  characters  not  found  in  any  but  remote 
parents  may  be  accounted  for  by  the  presence  of  recess- 
ive allelomorphs  which  have  not  until  the  present 
generation  been  able  to  escape  the  dominance  of  the 
opposed  character. 

Mendel's  discoveries  regarding  dominance  are  of  the 
greatest  importance  to  every  student  of  the  problems 
of  heredity  and  are  well  synoptized  by  Castle  thus: 

1.  "The  offspring  of  two  parents,  differing  in  respect  of  one 
character,    all    resemble   one   parent   and    therefore   possess   the 
dominant  character,  that  of  the  other  parent  being  recessive  or 
latent. 

2.  "In  the  place  of  simple  dominance  there  may  be  manifest 
in  the  intermediate  hybrid  offspring  an  intensification  of  character 
or  a  condition  intermediate  between  the  two  parents;  or  the  off- 
spring may  have  peculiar  characters  of  their  own  (heterozygotes) . 

3.  "A  segregation  of  the  characters  united  in  the  hybrid  takes 
place  in  their  offspring  so  that  a  certain  per  cent,  of  these  offspring 
possess  the  dominant  character  alone,   a  certain  per  cent,   the 
recessive  character  alone,   while  a  certain   per  cent,   are  again 
hybrid  in  nature." 

When  the  attempt  is  made  to  follow  a  number  of  Men- 
delian  characters  at  the  same  time,  the  whole  matter 
becomes  extremely  complicated. 

When  hybrids  result  from  combinations  of  totally 
different  species  they  are  usually  infertile,  have  no 
progeny,  and  so  die  without  affording  an  opportunity 
for  the  Mendelian  law  to  be  operative. 

It  is  sometimes  difficult  to  prejudge  what  characters 
may  be  subject  to  the  Mendelian  law.  Thus  whiteness 
and  color  are  Mendelian  characters  among  most  plants 


PLATE    I 


Mendelian  proportions  in  maize.  Cobs  born  by  heterozygote  plants 
pollinated  with  the  recessive,  showing  equality  of  smooth  and  wrinkled 
and  of  colored  and  white  grains  (Lock). 


CONFORMITY   TO   TYPE 


273 


and  among  the  lower  animals,  but  whiteness  and  black- 
ness of  the  human  skin  are  not  Mendelian.  On  the 
other  hand,  the  color  of  the  human  eyes  appears  to  be 
a  Mendelian  character.  Many  spontaneously  appearing 
pathological  conditions  are  Mendelian,  -as,  for  example, 
polydactylism  or  excessive  fingers  and  toes,  hemophilia 
or  bleeder's  disease,  deaf-mutism,  color-blindness,  etc. 
The  frequency  with  which  generations  are  skipped  in  the 
heredity  of  such  conditions  is  fully  explained  by  the  laws 
of  dominance  and  recession. 

DIAGRAM     EXPLAINING    MENDELIAN    PROPORTIONS. 

For  purposes  of  convenience  we  imagine  two  organisms,  one  the 
female  (9),  black;  the  other  the  male,  (<f),  white,  whose  cells  have 
but  two  chromosomes. 

9   Black  animal.  <?  White  animal. 

Resting  germinal  cells. 

Primary  Oocytes.  —  Primary  Spermato- 

cyte. 


Preparation  for  reduction. 
Appearance  of  the  chromosomes. 


Conjugation  of  the  chromosomes. 


Heterotype  mitosis  with  reduction  of  the  . 
chromosomes.    (Reduction  Division.) 


Results  of  the  Reduction  Division. 

-Ovum  and  Polar  body — Secondary  sperm- 

atocytes. 


Second  Division  by  Homotype   Mitosis. 


Outcome  of  the  Second  Division. 

Ovum  and  three  polar     Four   spermato- 

bodies.     The    ovum       zoa  all  equally 

alone  physiologically       physiologically 

functional.  functional. 


274  BIOLOGY:  GENERAL  AND  MEDICAL 

When  the  ovum  thus  matured  is  fertilized  by  one  of  the  sperma- 
tozoa thus  evolved,  the  result  is  an  organism  whose  somatic  and 
germinal  cells  must  all  have  a  chromosome  combination  diagram - 
matically  represented  thus — 

Fi  =  lst  filial  generation  (Hybrid). 


Taking  male  and  female  organisms  with  germ  plasm  of  this  com- 
bination, it  follows  that  in  the  reduction  or  heterotype  division  of  the 
oocyte  and  spermatocyte,  a  separation  of  these  chromosomes  will  be 
effected. 

Four  oocytes  are  intro-  /^~**y/~~*\ 
duced  because  there  will  /  ff  \l  f  \ 
be  four  spermatozoa.  V  j  \  J 


In  the  homotype  mitosis  these  arrangements 
will  not  be  changed,  and  the  results  will  be 


F2=2d  filial   generation,  resulting  from  the  fertilization  of  the  ova  of  hybrids 
by  the  spermatozoa  of  hybrids.     The  following  possibilities  are  shown: — 

9  9 

^ 


\/  ^X  \X  V>^ N/  V^          ^/  ^-^ 

Purewhiteg)  (g)- Hybrids g)  (J)  Pure  black, 


Mendelian  proportions. 
FIG.  104. 

One  of  the  most  recent  theories  of  heredity,  meriting 
attention,  is  a  chemico-physical  theory  which  may  be 
described  as  the  Lateral  Chain  Theory  of  Adami. 

In  his  "Principles  of  Pathology"  Adami  introduces 
and  explains  the  theory  as  follows: 

"  We  have  already  laid  down  that  the  primordial  living  matter 
of  the  cell  is  contained  in  the  nucleus;  it  is  this  matter  that  must  be 
carried  over  in  the  chromosomes.  From  this  it  follows  that  our 
theory  must  be  expressed  in  terms  of  biophoric  molecules,  and 
that  we  have  to  endeavor  to  conceive  a  constitution  of,  and  mode 
of  introduction  between  these  biophores  from  the  two  parental 
germ  cells  which  will  satisfy  the  various  conditions.  Coming 
now  "to  an  analysis  of  the  different  forms  of  inheritance,  we  may 


CONFORMITY  TO   TYPE  275 

make  out  that  a  particular  feature  showing  itself  in  either  parent 
may: 

A.  Present  itself  also  in  the  offspring: 

1.  Dominant,   wholly  replacing  the  corresponding  but 
divergent  feature  seen  in  the  other  parent. 

2.  Blended,  this  particular  feature  in  the  offspring  being 
intermediate  in  character  between  that  exhibited  in 
the  two  parents. 

3.  In  mosaic  form,  in  certain  cells  the  paternal,  in  others 
the  maternal  feature  being  dominant. 

4.  Blended  and  excessive,  the  features  being  more  pro- 
nounced than  in  either  parent. 

B.  Be  unrecognizable  in  the  offspring: 

1.  Recessive,  and  replaced  by  the  corresponding  feature 
derived  from  the  other  parent,  but  as  such  latent, 
capable  of  reappearing  in  later  generations. 

2.  Absent,  wholly  wanting  in  subsequent  generations,  the 
absence  being  due  either: 

a.  To  casting  out  of  an  inherited  condition,  or 

b.  To  the  feature  seen  in  the  parent  being  an  ac- 
quirement and  not  an  inheritance. 

"  On  the  other  hand,  considering  the  individual,  we  note  that  as 
regards  any  particular  feature  or  group  of  features  there  may  be: 

A.  Normal  Inheritance. — The  offspring  not  being  in  this  respect 
advanced  beyond  either  parent,  but  at  the  same  time  not  fallen 
behind. 

B.  Progressive    Inheritance. — The    offspring    being    advanced 
beyond  the  more  advanced  of  the  two  parents  and  exhibiting 
either: 

1.  Excessive  development  of  the  condition  or  conditions 
already  observable  in  one  or  both  parents,  or 

2.  Spontaneous  variations  (mutation) ;  i.e.,  the  appearance 
of  conditions  not  previously  noted  in  either  parent  or 
either  parental  stock. 

C.  Retrogressive    or    Reversionary    Inheritance. — The    offspring 
reverting  as  regards  any  feature  or  group  of  features  to  a  lower 
stage  in  the  phylogeny  of  the  species. 

D.  Non-inheritance. — Apparent  or  actual. 

"From  this  analysis  one  thing  at  least  is  obvious,  namely,  that 
the  biophores  derived  from  either  parent  are  liable  to  retain  their 
identity  for  some  generations.  Or,  to  be  more  accurate,  that 
qualities  conveyed  by  the  parental  biophores  may  be  retained, 
even  if  in  a  recessive  or  latent  condition. 

"That,  indeed,  is  clearly  proved  by  the  Mendelian  studies  on 
hybridization:  after  six  generations  or  more  with  self-fertilization 
the  hybrid  can  give  origin  to  plants  exhibiting  the  pure  features 
of  either  the  dominant  or  recessive  ancestor.  Conjugation  cannot, 
therefore,  be  of  the  nature  of  a  chemical  union  of  the  biophores 
from  the  two  sources  with  resultant  formation  of  a  new  biophoric 
substance.  On  the  other  hand,  we  cannot  conclude  that  all  the 
separate  biophores  contributed  by  and  representing  each  ancestor 
are  potentially  present  in  the  fertilized  ovum.  This  would 


276  BIOLOGY:  GENERAL  AND  MEDICAL 

demand  an  infinite  number.  The  existence  of  determinants,  such 
as  Weismann  conceived,  is,  as  we  have  pointed  out,  a  physical 
impossibility,  and  this  is  equally  so,  were  ten  generations  repre- 
sented that  would  demand  the  presence  in  each  chromosome  of 
more  than  one  thousand  separate  orders  of  biophores. 

"Our  way,  then,  lies  between  Scylla  and  Charybdis.  Still, 
between  those  two  the  cautious  mariner  could  advance  his  craft 
and,  the  gods  helping,  could  achieve  through  the  straits.  And 
here  we  would  urge  that  our  conception  of  the  constitution  of  the 
biophore  affords  us  a  proper  equipment  to  achieve  the  passage. 
We  have,  it  will  be  remembered,  been  led  onward  to  regard  the 
biophoric  molecules  as  composed  of  a  central  body  or  ring  of  nuclei 
provided  with  side  chains  which  are  dissociated  with  the  greatest 
ease.  As  the  environment  has  been  modified,  so  have  the  side 
chains  undergone  modification,  and  as  these  side  chains  become 
utilized  in  the  polymerization  of  the  biophoric  matter  and  the 
formation  of  new  biophores,  so  there  has  been  a  progressive  in- 
crease in  the  complexity  of  the  biophoric  molecule. 

"  We  have  pointed  out  how,  neglecting  determinants,  we  must 
regard  the  biophores  in  the  somatic  cells  as  undergoing  extensive 
modification  when  their  environment  has  become  altered,  whereby 
they  have  given  rise  to  or  controlled  the  different  orders  of  cells  in 
the  different  tissues.  As  regards  the  germ  cells,  their  biophores 
must  similarly  be  influenced,  for  it  is  upon  their  modification  that 
the  whole  evolution  of  living  forms  has  depended.  Clearly,  the 
biophores  of  the  human  ovum  are  vastly  more  complicated  than 
those  of  the  amoeba  or,  again,  than  those  of  the  lowest  multicellular 
organism  of  the  line  of  man's  ascent,  and  yet  the  progressive  elabora- 
tion of  the  soma  or  body  throughout  the  course  of  the  ascent  has 
been  the  outcome  of  the  germ  plasm  and  the  biophores  of  the 
same  within  ovary  and  testis. 

"There  are  two  or  three  possible  causes  for  the  progressive 
variations  of  multicellular  organisms:  the  mingling  of  the  germ 
plasms  in  conjugation  (amphimixis),  the  effect  of  environment  on 
the  respective  germ  plasms,  and  the  effect  of  both  of  these  com- 
bined. The  first  of  these  was  strenuously  upheld  for  long  by 
Weismann  as  the  controlling  cause,  but  he  was  compelled  to 
admit  that  the  second  must  also  be  in  action.  In  regard  to  this 
second  cause,  we  have  demonstrated  that  it  is  clearly  in  action 
in  unicellular  organisms  that  do  not  conjugate,  as  also  in  the 
somatic  cells  of  the  highest  multicellular  forms  of  life;  it  is  illogical 
to  deny  its  action  upon  the  germ  cells  of  the  same.  Not  to  waste 
time  by  taking  part  in  what  has  been  an  angry  discussion,  we  are 
prepared  to  accept  the  third  course — to  admit  that  both  the  action 
of  external  agencies  and  amphimixis  are  factors  in  variation, 
retrogressive  as  well  as  progressive. 

"Granting  this,  and  admitting  that  through  the  action  of  both 
it  comes  to  pass  that  the  germinal  biophores  in  no  two  members  of 
the  same  species  are  absolutely  alike  in  constitution,  what  must 
we  conceive  to  be  their  action  upon  each  other  when,  through 
conjugation,  biophores  of  two  orders  come  together  in  the  same 
cell,  the  fertilized  ovum? 


CONFORMITY  TO    TYPE  277 

"The  facts  of  inheritance  and  what  we  know  regarding  its 
histological  basis  entirely  refute  the  hypothesis  that  the  biophore 
molecules  as  a  whole  undergo  chemical  union.  We  may  therefore 
conceive  these,  in  the  first  place,  as  lying  side  by  side  in  a  common 
cytoplasm  or,  to  be  more  exact,  nuclear  sac,  in  the  process  of 
assimilation  attracting  ions  in  the  surrounding  medium,  building 
these  up  into  side  chains  of  different  orders.  Of  these  side  chains 
some  are  identical — common,  that  is,  to  the  molecules  of  both  sets 
of  biophores — some,  on  the  other  hand,  of  unlike  constitution,  so 
that  certain  side  chains  having  corresponding  position  or  attach- 
ments in  the  two  sets  of  parental  biophores  are  dissimilar.  As 
demonstrated  by  studies  upon  immunity,  we  regard  such  side 
chains  as  detachable  and  apt  to  be  detached,  that  is,  to  be  devel- 
oped in  excess,  and  then  becoming  loose,  passing  into  the  sur- 
rounding cytoplasm.  Again,  as  we  have  pointed  out,  we  must 
regard  growth  and  increase  in  the  number  of  biophores,  as  brought 
about  in  the  first  instance  by  the  building  up  of  nuclei  or  side-chain 
matter,  this  matter  attracting  other  matter  in  due  order,  so  that 
gradually  new  rings  are  constituted — new  biophores.  If  these 
views  be  correct,  then,  when  the  molecules  of  closely  allied  con- 
stitution and  properties  are  growing  side  by  side,  what  is  there 
in  this  process  to  determine  that  side-chain  matter  which  has  been 
liberated  under  the  influence  of  one  set  of  biophores  and  has 
become  detached  does  not  become  attracted  to  and  built  up  into 
the  substance  of  the  'growing  biophores'  of  the  other  set?  I 
cannot  but  hold  that  under  these  conditions— that  is,  conditions 
under  which  we  have  compound  molecules  of  very  similar  structure 
becoming  built  up  side  by  side — this  must  inevitably  occur  in  a 
common  fluid  medium. 

"  Whenever  a  greater  affinity  exists  between  the  components  of 
one  growing  biophore  and  certain  side-chain  nuclei  developed  under 
the  influence  of  the  molecules  of  the  other  set  of  biophores,  then 
these  nuclei  will  be  apt  to  be  built  into,  to  become  an  integral  part 
of,  the  new  biophores,  to  the  exclusion  of  the  corresponding  nuclei 
— those  proper  to  the  original  molecules.  In  short,  there  will  be, 

Ehysically  speaking,  a  contest  between  the  two  orders  of  growing 
iophores,  and,  to  a  certain  degree,  a  selection  or  rearrangement 
of  constituent  nuclei.  This  rearrangement  in  the  simplest  case 
will  result  in  an  interchange  of  constituent  parts;  in  other  cases 
may  result  in  side-chain  material  derived  from  one  parental 
biophore,  and  possessing  powerful  affinities  to  the  growing  bio- 
phores of  both  orders,  becoming  built  up  into  both  sets,  to  the 
exclusion  of  corresponding  but  weaker  side  chains  (so  that  these 
become  wholly  cast  out)  and  with  this  the  properties  determined 
by  their  presence  disappear  in  the  next  generation.  In  other  cases, 
again,  we  can  premise  an  interaction  between  certain  side-chain 
groups  derived  from  the  two  parental  biophores,  the  resultants 
of  this  interaction  becoming  built  up  into  the  growing  biophores, 
the  interaction  having  as  a  result  either  an  exaltation  or  a  depres- 
sion of  parental  character  or,  again,  leading  to  the  production  of 
mutation. 

"  Granted,  that  is,  that  in  its  broad  lines  we  have  come  to  realize 


278 


BIOLOGY:    GENERAL    AND    MEDICAL 


the  mode  of  constitution  of  the  proteidpgenous  molecule  and  that 
we  are  justified  in  assuming  that  the  biophoric  or  living  molecules 
partake  of  similar  constitution  and  that  our  conception  of  growth 
is  that  which  must  be  accepted,  then,  under  these  conditions, 
growth,  in  a  common  medium,  of  biophoric  molecules  of  two 
orders,  alike  in  general  constitution  but  differing  in  certain  of  their 
component  chemical  nuclei,  must  result  in  a  certain  amount  of 
interchange  of  those  nuclei.  Two  sets  of  biophores  may  still  be 
traced  in  the  blastomeres,  in  germ  cells,  and  other  cells  derived 
from  the  fertilized  ovum;  two  sets  each  derived  by  direct  physical 
descent  from  the  original  paternal  and  maternal  biophores  and 


V— d 


Fro.  105. — Schema  of  mode  of  interaction  of  two  biophoric  molecules  in  a 
common  cell  sap:  A,  of  maternal;  B,  of  paternal  origin.  1,  2,  3,  Allelomorphic 
Bide  chains,  which,  when  liberated  into  the  cell  sap,  will  be  attracted  to  the 
biophore  exercising  the  strongest  affinity;  4,  side  chains  common  to  both  mole- 
cules, built  up  indifferently  into  either.  (Adami,) 

chromosomes,  respectively,  but  the  members  of  each  of  these, 
while  building  up  into  their  structure  material  assimilated  by  their 
legitimate  progenitors,  attract  for  purposes  of  growth  allelomorphic 
matter  formed  similarly  by  the  other. 

"  By  this  method,  apart  wholly  from  what  may  be  regarded  as 
external  influences  acting  upon  the  germ  cells  during  their  existence 
within  the  organism  of  the  individual,  it  must  come  to  pass  that 
through  conjugation  the  biophores  giving  rise  to  a  new  individual 
are  not  identical  with  those  of  either  parent,  and  that  each  comes 
to  lose  certain  properties  which  belonged  to  the  biophores  of  the 
one  and  gain  some  belonging  to  the  biophores  of  the  other.  If 


CONFORMITY    TO   TYPE  279 

this  be  so,  then  we  can  picture  that  in  the  process  of  reduction  and 
casting  out  of  biophoric  material  in  the  development  both  of  the 
oocyte  and  of  the  spermatocyte,  while  there  are  delivered  to  the 
ovum  molecules  of  living  matter  which  in  direct  descent  have  been 
derived  from  one  parent  only,  those  molecules  may  convey  to  the  ovum 
constitution  and  properties  which  have  been  derived  from  both 
parents.  In  this  way,  without  any  increase  in  the  number  of 
determinants  or  ids,  by  this  chemical  modification  of  biophores,  a 
constant  number  of  such  biophoric  molecules  may  become  the 
bearers  of  properties  derived  from  a  long  series  of  ancestors. 

"We  purposely  do  not  here  consider  all  the  different  types  of 
inheritance,  for  this  is  not  a  full  treatise  upon  the  subject.  We 
have  taken  up  forms  that  are  sufficiently  wide  apart  to  show  that 
this  biophoric  theory  is  capable  of  elucidating  their  occurrence. 

"  It  appears  to  us  to  have  the  great  advantage  of  explaining  how 
hereditary  characters  may  be  conveyed  through  a  relatively  small 
number  of  molecules  of  highly  complex  organization;  how  those 
molecules  can  in  the  course  of  amphimixis  undergo  modification 
through  interaction;  how  they  can  become  modified  through  the 
action  both  of  amphimixis  and  of  environment;  how  similarly  they 
may  undergo  retrogressive  changes  and  lose  certain  properties 
under  the  same  influences. 

"With  reference  to  the  action  of  environment  on  the  germinal 
biophores  it  is  still  necessary  that  something  be  said,  but  our 
treatment  of  the  subject  of  amphimixis  will  not  be  complete  with- 
out reference  to  the  remarkable  reduction  process  that  precedes 
fertilization.  The  mode  of  that  reduction  we  have  already 
described.  We  have  seen  that  in  the  process  of  the  maturation 
of  the  ovum,  three-quarters  of  the  chromatin  present  in  the 
penultimate  stage  of  the  process  is  cast  out  (three  polar  bodies), 
one-quarter  only  being  retained,  and  that  similarly  the  sperma- 
tozoon is  developed  from  one-quarter  of  the  nuclear  matter  of  the 
primary  spermatocyte. 

"As  shown  by  the  abundant  recent  studies  on  Mendelism,  the 
results  of  this  reduction  may  be  very  remarkable;  certain  proper- 
ties may  at  a  single  conjunction  be  thrown  out  so  completely  that 
they  do  not  reappear  in  subsequent  generations. 

"  During  the  very  first  process  of  reduction  in  a  hybrid  a  property 
or  properties  derived  from  the  one  parent  may  thus  be  thrown 
out;  and  yet  when  the  parents  had  differed  in  several  particulars, 
at  this  same  moment  properties  derived  from  the  other  parent 
may  likewise  disappear.  And  as  in  such  hybridization  there  may 
be  as  many  as  a  score  of  properties  in  which  the  two  parents  had 
been  contrasted — size,  color  of  flower,  position  of  flowers,  shape  of 
leaf,  hairiness  of  leaves,  shape  of  seed,  etc., — the  process  of 
sorting  prior  to  this  casting  out,  if  we  regard  these  qualities  as 
conveyed  by  distinct  ids  or  determinants,  is  beyond  conception. 
It  demands  so  exact  a  localization  in  each  chromosome  ot  the 
particular  determinants,  and  at  the  same  time  so  precise  a  dis- 
tribution of  the  determinants  for  the  various  properties,  that  by  no 
possible  means  have  we  been  able  to  visualize  what  is  supposed  to 
happen.  By  the  biophoric  concept  this  casting-out  process  is, 


280 


BIOLOGY:  GENERAL  AND  MEDICAL 


we  think,  comprehensible,  namely,  as  has  already  been  stated, 
we  can  imagine  that  during  the  sojourn  together  of  the  parental 
biophores  in  the  germ  cells  of  the  new  individual,  from  the  moment 
of  fusion  of  the  parental  germ  plasms  to  give  rise  to  that  individual, 

cocrrzs  (HYBRID), 
giving  rise  each  to  one  fount  tly  reduction. 


V&M  MOTHER  CELL  (WBRI0), 

rise  to  four 
atozoa.. 


Dominant.  tybrid.  tybrid.  fiecessit/e. 

DD  JDK  DK  fift 

FIG.  106. — Schema  to  illustrate  Mendel's  law  regarding  the  second  hybrid 
generation  as  regards  a  single  pair  of  features,  as  also  to  illustrate  the  effects  of 
reduction  of  the  chromosomes  in  oogenesis  and  spermatogenesis. 

Each  germ  cell  (first  row)  is  originally  provided  with  chromosomes  of  paternal 
(black)  and  of  maternal  origin  (white).  The  existence  of  the  law  demands  that 
in  the  process  of  reduction  the  ovum  and  the  spermatozoon  (second  row)  become 
provided  with  chromosomes  (and  biophores)  that  are  of  either  paternal  or  of 
maternal  descent,  but  not  of  both;  although,  as  above  noted,  the  biophores  may 
in  their  growth  and  development  have  attracted  side  chains  formed  primarily 
by  the  opposed  order  of  biophores,  to  the  exclusion  of  those  originally  belonging 
to  them.  (Adami.) 

up  to  the  maturation  of  his  or  her  germ  cells,  there  is  an  interaction 
and  interchange  between  the  side  chains  to  whose  presence  is  due 
these  contrasted  features,  and  this  of  such  a  nature  that  the  newly 
developed  biophores,  descended,  let  us  say,  from  the  biophores 


CONFORMITY   TO   TYPE  281 

of  the  female  parent,  have  not  the  identical  composition  of  those 
parental  biophores.  In  the  process  of  growth  and  formation  there 
has  been,  as  it  Were,  a  selective  process. 

"Owing  to  the  greater  affinities,  they  have  attracted  and  built 
unto  themselves  certain  side  chains  derived  from  the  paternal 
biophores,  and  from  merely  attracting  them  in  the  first  place, 
they  have  come  to  form  them  actively.  According  to  our  con- 
ception, that  is,  a  side  chain,  to  whatever  central  ring  it  is  attached, 
tends  to  attract  ions  and  radicals  of  a  particular  order  to  itself,  so 
as  to  reproduce  itself  in  series.  This  interchange  depending  upon 
chemical  affinities  will  not  be  universal,  affecting  all  the  side- 
chains  of  both  paternal  and  maternal  biophores;  the  newly  formed 
biophores  will  present  an  admixture  of  the  two  orders;  they  will 
occupy  definite  positions  in  the  nuclear  thread  and  in  the  chromo- 
somes derived  from  that  thread. 

"Thus  it  will  happen  that  in  the  process  of  reduction,  as  in- 
dicated by  the  studies  upon  hybridization,  the  maturing  ovum,  or 
the  spermatozoon,  may  come  to  contain  biophores  of  paternal  or 
purely  maternal  origin.  The  accompanying  diagram  indicates 
what  we  conceive  to  be  the  process. 

"Along  these  lines  we  believe  it  is  possible  to  conceive  the 
conveyance  of  a  limited  number  of  biophores  in  the  germ  cells, 
from  generation  to  generation,  those  biophores  under  favorable 
conditions  gaining  through  amphimixis  accretions  to  their  proper- 
ties, under  favorable  conditions  becoming  shorn  of  certain  proper- 
ties, and  as  a  result  the  individuals  developing  from  these  germ 
cells  may  show  either  progressive  evolution  or  devolution." 

REFERENCES. 

HERBERT  SPENCER:     "Principles  of  Biology,"  1864,  N.  Y.,  1891. 

CHARLES  DARWIN:  "The  Variation  of  Animals  and  Plants  under 
Domestication,"  New  York,  1897 

FRANCIS  GALTON:  "  A  Theory  of  Heredity,"  Jour,  of  the  Anthrop- 
ological Institute,  1875.  "Hereditary  Genius,"  N.  Y., 
1870.  "  Inquiry  into  the  Human  Faculty." 

AUGUST  WEISMANN:     "The  Germ  Plasm,"  N.  Y.,  1898. 

W.  K.  BROOKS:     "The  Laws  of  Heredity,"  etc.,  Baltimore,  1883. 

GREGOR  JOHANN  MEXDEL:  "Versuche  uber  Pflanzenhybriden," 
1865.  Translation  published  in  the  Journal  of  the  Royal 
Horticultural  Society  of  London,  1901-2. 

W.   BATESON:     "Mendel's  Principles   of    Heredity,"  Cambridge, 

YVES    DELAGE:     "L'heredite  et    les    grandes    problemes    de    la 

biologic,"  Paris,  1903. 

R.  C.  PUNXETT:     "Mendelism,"  Cambridge,  1909. 
.ROBERT  HEATH  LOCK:   "Recent  Progress  in  the  Study  of  Vari- 
ation, Heredity  and  Evolution,"  London,  1909. 


CHAPTER  XT. 
DIVERGENCE. 

The  classification  or  orderly  arrangement  of  living 
things  according  to  structural  simplicity  and  complexity 
seems  to  have  been  early  followed  by  the  deduction 
that  the  simpler  forms  appearing  first,  the  complex 
forms  descended  from  them  by  evolution.  Such  ideas 
are  very  old  and  can  be  found  in  the  Greek  philosophy 
of  nearly  two  thousand  five  hundred  years  ago. 

Anaximander  of  Miletus  (B.  c.  611),  his  disciple 
Anaximenes  (528  B.  c.),  Heraclitus  of  Ephesus,  Pythag- 
oras of  Samas  (B.  c.  582),  Parmenides  of  Elea  (B.  c. 
515)  and  Empedocles  of  Agrigentum  (B.  c.  500  (?)), 
all  busied  themselves  with  cosmical  speculation  and 
offered  various  evolutionary  hypotheses  for  the  genesis 
of  our  planet.  Empedocles  went  a  step  farther  and 
speculated  upon  the  origin  of  the  living  beings  that 
people  the  earth.  He  believed  that  "plants  first  sprang 
from  the  earth  while  the  latter  was  still  in  process  of 
development.  After  them  came  the  animals,  their 
different  parts  having  first  formed  themselves  independ- 
ently and  then  been  joined  by  love;  subsequently  the 
ordinary  method  of  reproduction  took  the  place  of  this 
original  generation.  At  first  eyes,  arms,  etc.,  existed 
separately;  as  the  result  of  their  combination  arose  many 
monstrosities  which  perished;  those  combinations  which 
were  capable  of  subsisting,  persisted  and  propagated 
themselves." 

Aristotle  of  Stageiros  in  Thrace  (384  B.  c.),  the  first 
of  the  physiologists,  taught  vaguely  that  there  was  a 
gradual  succession  of  life  forms  from  the  less  to  the 
more  perfect,  but  seems  to  have  believed  that  they  were 
separate  creations. 

282 


DIVERGENCE  283 

With  the  coming  of  the  Christian  faith  speculation 
upon  the  ultimate  cause  of  things,  their  mode  of  origin 
and  the  order  of  their  succession  became  lost  through 
the  acceptance  of  the  Jewish  cosmogony  which  repre- 
sented the  world  and  all  its  organized  beings  as  having 
been  created  by  Yehwe  (Jehovah)  in  the  six  creation 
days. 

St.  Augustine  (A.  D.  354-430)  entertained  a  philo- 
sophical conception  of  creation  that  probably  grew  out 
of  his  early  Manichean  education,  and  speaks  of  the 
"creation  of  things  by  a  series  of  causes."  Thomas 
Aquinas  (1226-74)  recalled  St,  Augustine's  teaching  and 
upheld  it,  but  the  "creation"  became  a  dogma  of  the 
Church  and  interrupted  scientific  thought  and  investiga- 
tion for  many  centuries. 

In  tracing  the  history  of  the  evolutionary  hypothesis 
it  is  most  interesting  to  observe  that  it  reappeared  in  the 
Middle  Ages,  as  it  primarily  appeared  among  the  Greeks, 
in  the  writings  of  the  philosophers  and  not  in  those  of  the 
naturalists.  Thus  the  German  philosopher,  Leibnitz, 
(1646-1716)  conceived  that  living  beings  form  an 
unbroken  series  from  the  simple  to  the  complex,  some 
steps  in  the  series  having  become  extinct.  He  also 
conceived  that  individual  forms'  underwent  change  as 
the  result  of  the  action  of  external  forces,  and  believed 
that  Nature  was  progressive  and  ever  advancing.  The 
advance  is,  however,  slow,  hence  his  dictum,  "  Natura 
nonfacit  saltum." 

Thoroughly  imbued  with  the  teachings  of  Leibnitz, 
Buff  on  (1707-88)  continued  and  enlarged  the  thought. 
He  believed  that  organisms  could  be  modified  by  changes 
in  climate,  food,  and  domestication,  and  also  that  parts 
could  be  modified  by  disuse.  He  also  held  that  all 
animals  might  be  derived  from  a  single  type. 
,  The  philosopher  David  Hume  thought  that  "the 
world  might  have  been  gradually  produced  from  very 
small  beginnings,  increasing  by  the  activity  of  its  in- 
herent principles  rather  than  by  a  sudden  evolution 


284  BIOLOGY:  GENERAL  AND  MEDICAL 

of  the  whole  by  the  almighty  fire.  What  a  magnificent 
idea  of  the  infinite  power  of  The  Great  Architect!  The 
Cause  of  causes.  Parent  of  parents.  Ens  entium!" 

"De  Maillet,  writing  in  1753,  appears  to  have  been 
convinced  that  existing  species  of  animals  arose  through 
modification  of  their  predecessors.  At  the  beginning  of 
the  nineteenth  century  similar  speculations  were  pub- 
lished by  Goethe  and  by  Treviranus,  the  latter  having 
been  the  first  to  apply  the  term  '  Biology '  to  the  science 
of  the  phenomena  of  life." 

Erasmus  Darwin  (1731-1802),  an  English  physician 
and  naturalist,  wrote  upon  evolution,  embodying  his 
ideas  in  two  chief  works,  "The  Botanic  Garden"  (1789), 
and  "Zoonomia"  (1794-6).  He  believed  that  "all 
animals  have  originated  from  a  single  'living  filament'; 
that  changes  are  produced  by  differences  of  climate; 
that  all  animals  undergo  constant  changes,  and  that 
many  of  their  acquirements  are  transmitted  to  their 
posterity;  that  the  contests  of  the  males  for  the  possession 
of  the  females  lead  to  such  results  as  have  since  been 
stated  under  the  name  of  '  sexual  selection' ;  that  many 
structures  have  been  acquired  as  a  means  of  security  in 
a  struggle  for  existence;  and  that  a  vast  length  of  time 
has  elapsed  since  these  modifications  began." 

Following  Buffon,  who  was  his  personal  friend  and 
greatly  influenced  him,  came  the  first  of  the  modern 
evolutionists,  Jean  Baptiste  Pierre  Antoine  de  Monet 
de  Lamarck  (1744-1829),  well-educated,  a  capable  and 
versatile  naturalist,  well  versed  in  botany  and  zoology. 
He  greatly  improved  the  classification  of  animals,  being 
the  first  to  separate  vertebrates  and  invertebrates  and 
introduced  many  new  orders  into  the  classification.  He 
was  the  first  important  invertebrate  paleontologist,  and 
may  be  credited  with  having  founded  that  department 
of  science.  It  was  as  a  paleontologist  that  he  was  brought 
into  opposition  with  Cuvier  (1769-1832)  who  believed 
in  the  sudden  creation  and  extinction  of  species.  La- 
marck believed  that  the  fossil  forms  of  life  were  the 


DIVERGENCE  285 

ancestors  of  the  forms  now  living.  The  principles  to 
which  he  adhered  were:  "1.  The  great  length  of  geologi- 
cal time;  2.  The  continuous  existence  of  organic  life 
throughout  all  the  geological  periods;  3.  The  general 
similarity  of  the  physical  environment  throughout  the 
periods;  4.  Continual  gradual  changes  without  cata- 
clysms or  catastrophic  destruction  of  life;  5.  Gradual 
modifications  in  the  living  things  to  correspond  with 
the  environment;  6.  Gradual  modifications  of  the  habits 
to  coincide  with  the  gradual  changes  in  structure." 

Lamarck  believed  that  all  living  things  arose  from 
germs  that  developed  spontaneously,  and  that  the  most 
simple  of  these  was  "monad-like."  The  first  germs  of 
animal  and  vegetable  life  were  formed  in  favorable  places 
and  under  favorable  conditions.  The  functions  of  life 
beginning  and  an  organic  movement  being  established, 
these  germs  "necessarily  gradually  developed  into 
organs  so  that  after  a  time  and  under  suitable  circum- 
stances they  have  been  differentiated"  into  different 
parts  or  organs,  development  proceeding  from  the 
simple  to  the  complex.  The  time  during  which  such 
differentiations  have  been  in  progress  is  "absolutely 
beyond  the  power  of  man  to  appreciate  in  an  adequate 
way, "  but  "  With  the  aid  of  sufficient  time,  of  circum- 
stances which  have  necessarily  been  favorable,  of  changes 
of  condition  that  every  part  of  the  earth's  surface  has 
successively  undergone — in  a  word,  by  the  power  which 
new  situations  and  new  habits  have  of  modifying  the 
organs  of  living  beings — all  those  which  now  exist  have 
been  gradually  formed  as  we  now  see  them." 

In  the  progress  of  evolution  certain  factors  were 
looked  upon  as  essential;  these  are  now  known  as  La- 
marc  kian  factors  and  are: 

1.  "Favorable  circumstances  attending  changes  of  environment, 
soil,  food,  temperature,  etc.,  supposed  to  act  directly  in  the  case 
of  plants,  indirectly  in  the  case  of  animals  and  man. 

2.  "  Needs,  new  physical  wants  or  necessities  induced  by  the 
changed  conditions  of  life.     Lamarck   believed  that  change  of 
habits  may  lead  to  the  origination  or  modification  of  organs;  that 


286  BIOLOGY:  GENERAL  AND  MEDICAL 

changes  of  function  also  modify  or  create  new  organs.  By 
changes  of  environment  animals  become  subjected  to  new  sur- 
roundings, involving  new  ways  and  means  of  living.  Thus,  certain 
land  birds,  driven  by  necessity  to  obtain  their  food  in  the  water, 
gradually  assumed  characters  or  structures  adapting  them  for 
swimming,  wading,  or  for  searching  for  food  in  the  shallow  water, 
as  in  the  case  of  the  long-necked  kinds. 

3.  "Use  and  disuse.     To  use  an  organ  is  to  develop  it;  not  to 
use  it  is  to  eventually  lose  it.     The  anterior  limbs  of  birds  became 
capable  of  sustained  flight  through  use;  the  hind  limbs  of  whales 
are  lost  through  disuse,  etc. 

4.  "Competition.     Nature  takes  precautions  not  to  overcrowd 
the  earth.     The  stronger  and  larger  living  things  destroy  and 
devour    the    smaller   and    woaker.     The    smaller   multiply    very 
rapidly,  the  larger  slowly.     A  physiological  balance  is  maintained. 

5.  "The  transmission  of  acquired  characters.     The  advantage 
gained  by  every  individual  as  the  result  of  the  structural  changes 
resulting  from  use  or  disuse  are  handed  down  to  its  descendants 
who  begin  where  the  parent  leaves  off,  and  so  are  able  to  continue 
the  progression  or  retrogression  of  the  character. 

6.  "Cross-breeding.     'If  when  any  peculiarities  of  form  or  any 
defects   whatsoever  are  acquired,    the   individuals   in   this   case 
always  pairing,  they  will  produce  the  same  peculiarities,  and  if  for 
successive  generations  confined  to  such  unions,   a   special  and 
distinct  race  will  then  be  formed.     But  perpetual  crosses  between 
individuals  which  have  not  the  same  peculiarities  of  form  result 
in   the   disappearance   of  all   the  peculiarities  acquired   by   the 
particular  circumstances.' 

7.  "Isolation.     'Were    not    men    separated    by    distances    of 
habitation,  the  mixtures  resulting  from  crossing  would  obliterate 
the  general  characters  which  distinguish  different  nations.'     This 
thought  is  expressed  in  his  account  of  the  origin  of  man  from  apes, 
and  is  not  applied  to  living  things  in  general." 

Lamarck  sums  up  his  ideas  in  four  laws  published  in 
his  "  Animaux  sans  Vertebres,"  1815: 

I.  "Life,  by  its  proper  forces,  continually  tends  to  increase 
the  volume  of  every  body  which  possesses  it,  and  to  increase  the 
size  of  its  parts,  up  to  a  limit  which  brings  it  about." 

II.  "The  production  of  a  new  organ  in  the  animal  body  results 
from  the  supervention  of  a  new  want  which  continues  to  make 
itself  felt,  and  of  a  new  movement  which  this  want  gives  rise  to 
and  maintains." 

III.  "The  development  of  organs  and  their  power  of  action 
are  constantly  in  ratio  to  the  employment  of  these  organs." 

IV.  "Everything  which  has  been  acquired,  impressed  upon,  or 
changed  in  the  organization  of  individuals  during  the  course  of 
their  life  is  preserved   by  generation  and    transmitted   to  new 
individuals  which  have  descended  from  those  which  have  under- 
gone those  changes." 


DIVERGENCE  287 

This  last  law  contains  the  principle,  fundamental  in  his 
conception  of  evolution  for  which  Lamarck  is  best 
known,  viz. :  that  acquired  characters  are  transmitted  to  the 
offspring. 

While  Lamarck  was  thus  working  in  zoology  and 
paleontology,  an  Englishman,  Thomas  Robert  Malthus, 
was  working  upon  social  problems  and  evolving  truths 
that  were  destined  to  exert  a  profound  influence  upon 
some  who  were  to  follow.  In  1798  he  published  an  im- 
portant "  Essay  on  the  Principle  of  Population  as  it 
Affects  the  Future  Improvement  of  Society,"  of  which  a 
second  improved  edition  appeared  in  1803.  The  truth 
discovered  by  Malthus  was  that  population  at  all  times 
has  a  tendency  to  outgrow  subsistence.  The  population 
increases  in  geometrical  progression,  the  means  of  sub- 
sistence in  arithmetical  progression,  so  that  it  is  only 
starvation  that  keeps  the  population  in  check.  The  only 
means  of  preventing  overpopulation  is  moral  restraint. 

The  cogent  reasoning  in  the  essay  resulted  in  the 
introduction  of  a  new  principle — the  Malthusian  doctrine 
— into  economics  and  led  to  considerable  modification  in 
the  poor-laws  of  England. 

Lamarck's  work  and  teachings  were  eclipsed  by  the 
somewhat  bitter  opposition  as  well  as  brilliant  work  of 
his  compatriot,  Cuvier,  and  were  neglected  for  many 
years  when  they  were  revived  by  the  Neo-Lamarckians 
who  arose  in  an  endeavor  to  refute  Darwin. 

In  1844,  Robert  Chambers  published,  without  any 
name  upon  the  title  page,  a  little  book  entitled  "  Ves- 
tiges of  the  Natural  History  of  Creation"  that  fore- 
shadowed the  cosmical  evolution  to  be  popularized  by 
Herbert  Spencer.  The  book  was  republished  in  1846, 
but,  though  apparently  widely  read,  met  with  so  much 
opposition  from  religious  quarters  that  it  has  almost 
been  forgotten.  "  The  book  was  not  primarily  designed, 
as  many  have  intimated  in  their  criticisms  and  as  the 
title  might  be  thought  partly  to  imply,  to  establish 
a  new  theory  respecting  the  origin  of  animate  nature; 


288  BIOLOGY:  GENERAL  AND  MEDICAL 

nor  are  the  chief  arguments  directed  to  that  point.  The 
object  is  one  to  which  the  idea  of  an  organic  creation  in 
the  manner  of  natural  law  is  only  subordinate  and 
ministrative,  as  are  likewise  the  nebular  hypothesis  and 
the  doctrine  of  a  fixed  natural  order  in  mind  and  morals. 
The  purpose  is  to  show  that  the  whole  revelation  of  the 
works  of  God,  presented  to  our  senses  and  reason,  is  a 
system  based  on  what  we  are  compelled,  for  want  of  a 
better  term,  to  call  Law;  by  which,  however,  is  not 
meant  a  system  independent  or  exclusive  of  Deity,  but 
one  which  only  proposes  a  certain  mode  of  his  working." 

In  1852,  Herbert  Spencer,  the  philosopher  of  science, 
deduced  cosmical  evolution  by  philosophical  speculation, 
laid  the  foundation  of  his  future  great  work,  the  "Syn- 
thetic Philosophy,"  and  revived  an  interest  in  the  sub- 
ject, which,  now  that  it  was  supported  by  a  great  array 
of  scientific  fact,  began  to  take  a  firm  hold  upon  the 
thought  of  his  time. 

But  the  man  whose  life  and  work  are  most  completely 
identified  with  organic  evolution  is  Charles  Darwin 
(1809-82),  who,  having  spent  the  early  years  of  his  life  in 
travels,  during  which  he  had  exceptional  opportunities 
for  scientific  observation,  and  many  years  thereafter  in 
patient  study  and  experimentation,  in  1859  published 
an  epoch-making  work  upon  "The  Origin  of  Species  by 
Means  of  Natural  Selection,  or  The  Preservation  of  the 
Favored  Races  in  the  Struggle  for  Life." 

It  is  extremely  interesting  to  note  that  at  the  very 
time  at  which  Darwin  was  engaged  upon  this  work 
and  had  explained  it  to  his  friends,  to  whom  some  of  the 
sheets  were  shown  or  read,  another  was  working  in  the 
same  field  in  much  the  same  way  and  anticipated  him 
in  the  publication.  This  was  Alfred  Russell  Wallace, 
another  English  naturalist,  who,  curiously  enough,  had 
traveled  over  much  the  same  ground  that  Darwin  had 
covered  and  described  in  his  book,  "  Voyage  of  a  Natur- 
alist in  H.  M.  S.  Beagle,"  and  then  continued  his  travels 
to  the  East  Indies.  In  September,  1855,  he  published 


DIVERGENCE  289 

an  essay  "On  the  Law  which  has  Regulated  the  Intro- 
duction of  New  Species."  In  February,  1858,  he  wrote 
a  famous  essay  "  On  the  Tendency  of  Varieties  to  Depart 
Indefinitely  from  the  Original  Type."  The  appearance 
of  this  essay  led  to  the  publication  of  a  preliminary 
essay  by  Darwin,  and  the  papers  of  both  authors  were 
published  in  the  Proceedings  of  the  Linnean  Society  of 
London,  August,  1858.  To  the  credit  of  Darwin  it 
should  be  said  that,  finding  himself  anticipated  by  a 
friend,  he  expressed  his  complete  willingness  to  withdraw 
from  the  field,  but  was  dissuaded  from  pursuing  this 
course  by  the  friends  who  knew  the  wealth  and  value 
of  the  material  he  had  collected.  Wallace  parallels 
Darwin  in  discussing  the  nature  of  varieties,  the  struggle 
for  existence,  the  law  of  perpetuation,  of  useful  and  use- 
less variations,  and  the  partial  reversion  of  domesticated 
varieties,  but  though  his  writings  contain  the  same 
fundamental  thoughts,  Wallace  did  not  support  them 
with  the  cogency  and  thoroughness  of  Darwin  and  so 
has  been  eclipsed  by  the  greater  light. 

Darwin  taught  that  the  origin  of  species  depends 
upon  a  number  of  factors  which  may  be  summarized  as 
follows: 

1.  Over-production.    All  plants  and  animals  pro- 
duce more  offspring  than  can  possibly  survive 
under  natural  conditions. 

2.  Variation. — Among  the  offspring  no  two  are 
precisely  alike,  and  often  there  are  striking  dis- 
similarities, which  may  or  may  not  be  advanta- 
geous. 

3.  Struggle  for  existence. — Life  is  a  continual  con- 
test or  struggle  in  which  the  strongest  or  "fittest" 
survive.     Among  the  variations  of  every  species, 

.  those  better  adapting  the  individual  to  succeed 
in  the  struggle  for  existence  will  tend  to  be  per- 
petuated, those  unfitting  him  for  the  struggle, 
will  bring  about  extinction,  so  the  species  will 
tend  to  vary  and  new  species  gradually  arise. 

19 


290  BIOLOGY:  GENERAL  AND  MEDICAL 

4.  The  atrophy  and  gradual  disappearance  of  useless 
organs  and  appendages  in  species  that  survive  in 
the  struggle  for  existence. 

5.  Heredity. — The  characters  of  the  species  and 
special  characters  of  the  parents  reappear  in  the 
offspring,  thus  tending  to  preserve  those  varia- 
tions   that    prove    useful    in   the    struggle    for 
existence. 

6.  Sexual  selection. — The  contest  among  males  for 
the  possession  of  the  females,  in  which  the  poorer 
and  weaker  will  be   eliminated.     The  relative 
attractiveness  of  one  sex  for  the  other  results  in 
the  preservation  or  elimination  in  sexual  dimor- 
phism. 

The  chief  factor  he  considers  to  be  "natural  selection" 
or  "struggle  for  existence,"  which  he  finds  to  be  identical 
with  Herbert  Spencer's  doctrine  of  the  "Survival  of  the 
fittest." 

Darwin  begins  by  a  consideration  of  the  various 
"breeds"  of  domestic  animals  and  shows  that  from 
a  few  primitive  stocks  the  many  varieties  of  domestic 
animals  have  been  cultivated  by  artificial  selection.  He 
points  out  that  among  animals  there  are  slight  variations 
in  the  direction  of  desirability  and  undesirability,  and 
that  by  carefully  conserving  the  desirable  and  eliminat- 
ing the  undesirable,  man  has  been  able  to  produce  the 
various  kinds  of  cattle,  sheep,  hogs,  horses,  dogs,  rabbits, 
fowls,  pigeons,  etc.,  so  well  known  to  us.  If  it  is  to  be 
conceived  that  natural  selection  is  analogous  to  artificial 
selection,  it  is  first  necessary  to  admit  that  living  things 
o*'  the  same  kind  vary  under  natural  conditions.  Con- 
^.vnro&  urns,  he  says: 

"The  many  slight  differences  which  appear  in  the  offspring 
from  the  same  parents  or  which  it  may  be  presumed  have  thus 
arisen,  from  being  observed  in  the  same  locality,  may  be  called 
individual  differences.  No  one  supposes  that  all  the  individuals 
of  the  same  species  are  cast  in  the  same  actual  mould.  These 
individual  differences  are  of  the  highest  importance  to  us,  for  they 
are  often  inherited,  as  must  be  familiar  to  everyone;  and  they 
thus  afford  materials  for  natural  selection  to  act  on  and  accumulate 


DIVERGENCE  291 

in  the  same  manner  as  man  accumulates  in  any  given  direction 
individual  differences  in  his  domesticated  productions." 

"I  look  at  individual  differences,  though  of  small  interest  to 
the  systematist,  as  of  the  highest  importance  fog  us,  as  being  the 
first  steps  toward  such  slight  varieties  as  are  barely  thought 
worth  recording  in  works  on  natural  history.  And  I  look  at 
varieties  which  are  in  any  degree  more  distinct  and  permanent  as 
steps  toward  more  strongly  marked  and  permanent  varieties; 
and  at  the  latter  as  leading  to  sub-species,  and  then  to  species. 
The  passage  from  one  stage  of  difference  to  another  may,  in  many 
cases,  be  the  simple  result  of  the  nature  of  the  organism  and  of 
the  different  physical  conditions  to  which  it  has  long  been  exposed; 
but  with  respect  to  the  more  important  and  adaptive  characters, 
the  passage  from  one  stage  of  existence  to  another  may  be  safely 
attributed  to  the  cumulative  action  of  natural  selection,  hereafter 
to  be  explained,  and  to  the  effects  of  the  increased  use  or  disuse 
of  parts.  A  well-marked  variety  may  therefore  be  called  an 
incipient  species;  but  whether  this  belief  is  justifiable  must  be 
judged  by  the  weight  of  the  various  facts  and  considerations  to  be 
given  throughout  this  work"  [Origin  of  Species]. 

"Varieties  cannot  be  distinguished  from  species — except,  first, 
by  the  discovery  of  intermediate  linking  forms;  and,  secondly,  by  a 
certain  indefinite  amount  of  difference  between  them  for  two  forms, 
if  differing  very  little,  are  generally  ranked  as  varieties,  notwith- 
standing that  they  cannot  be  closely  connected;  but  the  amount  of 
difference  considered  necessary  to  give  to  any  two  forms  the  rank 
of  species  cannot  be  defined." 

"I  must  make  a  few  preliminary  remarks  to  show  how  the 
struggle  for  existence  bears  on  natural  selection.  It  has  been 
seen  .  .  .  that  among  organic  beings  in  a  state  of  nature  there  is 
some  individual  variability;  indeed  I  am  not  aware  that  it  has 
ever  been  disputed.  It  is  immaterial  for  us  whether  a  multitude 
of  doubtful  forms  be  called  species  or  sub-species  or  varieties  .  .  . 
if  the  existence  of  any  well-marked  varieties  be  admitted.  But 
the  mere  existence  of  individual  variability,  and  of  some  few 
well-marked  varieties,  though  necessary  as  the  foundation  for  this 
work,  helps  us  but  little  in  understanding  how  species  arise  in 
nature." 

"If  under  changing  conditions  of  life  organic  beings  present 
individual  differences  in  almost  every  part  of  their  structure,  and 
this  cannot  be  disputed;  if  there  be,  owing  to  their  geometrical 
rate  of  increase,  a  severe  struggle  for  life,  at  some  age,  season 
or  year,  and  this  certainly  cannot  be  disputed;  then,  considering 
the  infinite  complexity  of  the  relations  of  all  organic  beings  to 
each  other  and  to  their  condition  of  life,  causing  an  infinite  di- 
versity of  structure,  constitution  and  habits,  to  be  advantageous 
to  them,  it  would  be  a  most  extraordinary  fact  if  no  variations 
.'had  ever  occurred  useful  to  each  being's  own  welfare,  in  the  same 
manner  as  so  many  variations  have  occurred  useful  to  man. 
But  if  variations  useful  to  any  organic  being  ever  do  occur, 
assuredly  individuals  thus  characterized  will  have  the  best  chance 
of  being  preserved  in  the  struggle  of  life;  and  from  the  strong 


292  BIOLOGY:  GENERAL  AND  MEDICAL 

principle  of  inheritance  these  will  tend  to  produce  offspring  simi- 
larly characterized.  This  principle  of  preservation,  or  the  survival 
of  the  fittest,  I  have  called  natural  selection." 

"It  leads  to  the  improvement  of  each  creature  in  relation  to 
its  organic  and  inorganic  conditions  of  life,  and  consequently, 
in  most  cases,  to  what  must  be  regarded  as  an  advance  in  organ- 
ization. Nevertheless,  low  and  simple  forms  will  long  endure  if 
well  fitted  for  their  simple  conditions  of  life.  Natural  selection, 

at  corresponding  ages, 
the  adult.  Among 
its  aid  to  ordinary 
selection  by  assuring  to  the  most  vigorous  and  best  adapted  males 
the  greater  number  of  offspring.  Sexual  selection  will  also  give 
characters  useful  to  the  males  alone  in  their  struggles  or  rivalry 
with  other  males;  and  these  characters  will  be  transmitted  to  one 
sex  or  to  both  sexes,  according  to  the  form  of  inheritance  which 
prevails." 

"  But  we  have  already  seen  how  it  [natural  selection]  entails 
extinction;  and  how  largely  extinction  has  acted  in  the  world's 
history  geology  plainly  declares.  Natural  selection  also  leads  to 
divergence  of  character;  for  the  more  organic  beings  diverge  in 
structure,  habits,  and  constitution,  by  so  much  the  more  can  a 
large  number  be  supported  on  the  area,  of  which  we  see  proof  by 
looking  to  the  inhabitants  of  any  small  spot  and  to  the  productions 
naturalized  in  foreign  lands.  Therefore,  during  the  modification 
of  the  descendants  of  any  one  species,  and  during  the  incessant 
struggle  of  all  species  to  increase  in  numbers,  the  more  diversified 
the  descendants  become,  the  better  will  be  their  chance  of  success 
in  the  battle  for  life.  Thus  the  small  differences  distinguishing 
varieties  of  the  same  species,  steadily  tend  to  increase,  till  they 
equal  the  greater  differences  between  species  of  the  same  genus, 
or  even  of  distinct  genera." 

"It  is  the  common  and  widely  diffused  and  widely  ranging 
species  belonging  to  the  larger  genera  .within  each  class,  which 
vary  most;  and  these  tend  to  transmit  to  their  modified  offspring 
that  superiority  which  now  makes  them  dominant  in  their  own 
countries.  Natural  selection,  as  has  just  been  remarked,  leads  to 
divergence  of  character  and  to  much  extinction  of  the  less  improved 
and  intermediate  forms  of  life.  On  these  principles,  the  nature 
of  the  affinities  and  the  generally  well-defined  distinctions  between 
the  innumerable  organic  beings  in  each  class  throughout  the 
world,  may  be  explained.  It  is  a  truly  wonderful  fact — the 
wonder  of  which  we  are  apt  to  overlook  from  familiarity — that 
all  animals  and  all  plants  throughout  all  time  and  space  should  be 
related  to  each  other  in  groups,  subordinate  to  groups,  in  the 
manner  which  we  everywhere  behold — namely,  varieties  of  the 
same  species  most  closely  related,  species  of  the  same  genus  less, 
closely  and  unequally  related,  forming  sections  and  sub-genera, 
species  of  distinct  genera  much  less  closely  related  and  genera 
related  in  different  degrees,  forming  sub-families,  families,  orders, 
sub-classes  and  classes.  _  The  several  subordinate  groups  in  any 
class  cannot  be  ranked  in  a  single  file,  but  seem  clustered  round 


DIVERGENCE    '  293 

points,  and  these  round  other  points,  and  so  on  in  almost  endless 
cycles.  If  species  had  been  independently  created,  no  explanation 
would  have  been  possible  of  this  kind  of  classification;  but  it  is 
explained  through  inheritance  and  the  complex  action  of  natural 
selection,  entailing  extinction  and  divergence  of  character  as  we 
have  seen." 

"The  affinities  of  all  the  beings  of  the  same  class  have  some- 
times been  represented  by  a  great  tree.  I  believe  this  simile 
largely  speaks  the  truth.  The  green  and  budding  twigs  may 
represent  existing  species,  and  those  produced  during  former  years 
may  represent  the  long  succession  of  extinct  species.  At  each 
period  of  growth  all  the  growing  twigs  have  tried  to  branch  out  on 
all  sides,  and  to  overtop  and  kill  the  surrounding  twigs  and 
branches,  in  the  same  manner  as  species  and  groups  of  species 
have  at  all  times  overmastered  other  species  in  the  great  battle  for 
life.  The  limbs,  divided  into  great  branches,  and  these  into  lesser 
and  lesser  branches,  were  themselves  once,  when  the  tree  was 
young,  budding  twigs;  and  this  connection  of  the  former  and 
present  buds  by  ramifying  branches  may  well  represent  the 
classification  of  all  extinct  and  living  species  in  groups  subordinate 
to  groups.  Of  the  many  twigs  which  once  flourished  when  the 
tree  was  a  mere  bush,  only  two  or  three,  now  grown  into  great 
branches,  yet  survive  and  bear  the  other  branches;  so  with  the 
species  which  lived  during  long  past  geological  periods,  very  few 
have  left  living  and  modified  descendants.  From  the  first  growth 
of  the  tree  many  a  limb  and  branch  has  decayed  and  dropped  off; 
and  these  fallen  branches  of  various  sizes  may  represent  those 
whole  orders,  families  and  genera  which  have  now  no  living 
representatives  and  which  are  known  to  us  only  in  a  fossil  state. 
As  we  here  and  there  see  a  thin,  straggling  branch  springing  from 
a  fork  low  down  in  a  tree,  and  which  by  some  chance  has  been 
favored  and  is  still  alive  on  its  summit,  so  we  occasionally  see  an 
animal  like  the  Ornithorhynchus  or  Lepidosiren,  which  in  some 
small  degree  connects  by  its  affinities  two  large  branches  of  life, 
and  which  has  apparently  been  saved  from  fatal  competition  by 
having  inhabited  a  protected  station.  As  buds  give  rise  by  growth 
to  fresh  buds,  and  these,  if  vigorous,  branch  out  and  overtop  on 
all  sides  many  a  feebler  branch,  so  by  generation  I  believe  it  has 
been  with  the  great  Tree  of  Life,  which  fills  with  its  dead  and 
broken  branches  the  crust  of  the  earth,  and  covers  the  surface 
with  its  ever-branching  and  beautiful  ramifications." 

As  Mr.  Darwin  says  in  the  last  chapter  of  the  "  Origin 
of  Species":  "This  whole  volume  is  one  long  argument." 
It  is,  therefore,  difficult  to  give  it  the  force  it  should 
convey  either  through  a  series  of  excerpts,  such  as  has 
been  here  presented,  or  by  any  brief  synopsis  of  its 
contents.  To  read  the  book  is  to  become  impressed  by 
the  worth  of  the  argument  as  well  as  by  the  great  array 


294  BIOLOGY:  GENERAL  AND  MEDICAL 

of  carefully  chosen  facts  that  have  been  collected  in 
its  support.  Its  appearance  was  followed  by  an  enthu- 
siastic reception,  and  the  theory  of  natural  selection 
is  still  the  source  of  much  careful  examination  and 
experimentation. 

In  conclusion  Mr.  Darwin  makes  the  following  state- 
ment: "I  am  convinced  that  natural  selection  has  been 
the  main  but  not  the  exclusive  means  of  modification." 

In  one  of  the  excerpts  given  above  this  language  is 
used:  "  But  if  variations  useful  to  any  organic  being 
ever  do  occur,  assuredly  individuals  thus  characterized 
will  have  the  best  chance  of  being  preserved  in  the 
struggle  of  life;  and  from  the  strong  principle  of  in- 
heritance these  will  tend  to  produce  offspring  similarly 
characterized." 

If  the  readers  of  Darwin  had  followed  his  text  as  care- 
fully as  they  should,  some  of  the  errors  regarding  his 
opinions  might  have  been  escaped.  It  is  quite  clear 
that  he  believed  natural  selection  to  be  "the  main,  but 
not  the  exclusive  means  of  modification,"  and  it  is  equally 
evident  that  the  general  statement  that  his  theory 
hinges  upon  the  "transmission  of  acquired  characters" 
is  doubtfully  correct.  The  theory  really  treats  of  the 
preservation  of  useful,  and  the  elimination  of  useless 
characters  and  the  characters  themselves  appear  or  dis- 
appear spontaneously — i.e.,  as  the  result  of  the  natural 
tendency  of  living  things  to  vary.  The  origin  of  species 
according  to  Darwin's  conceptions  would  be  so  gradual 
as  to  be  imperceptible,  and  the  forces  by  which  the  new 
species  evolve  in  continuous  operation. 

The  first  effect  of  Darwin's  work  was  to  carry  the 
world  of  science  by  storm,  but  at  the  same  time  to  arouse 
intense  hostility  on  the  part  of  the  theologians  who 
found  the  theory  of  descent,  which,  as  has  been  shown, 
did  not  originate  with  Darwin,  incompatible  writh  the 
doctrine  of  Creation.  In  this  conflict,  Darwin  took 
little  part,  but  was  championed  by  Huxley,  while 
Bishop  Wilberforce  led  the  opposition.  The  battle  was 


DIVERGENCE  295 

long  and  bitter,  there  was  much  acrimonious  writing 
on  both  sides,  but  the  theory  of  descent — the  doctrine  of 
evolution — was  found  to  be  invulnerable  and  at  present 
the  theologians  themselves  have  accepted  it  and  even 
make  use  of  it  in  their  own  work. 

But  as  the  years  flew  by  the  Darwinian  doctrines 
began  to  meet  with  assaults  from  the  scientists  them- 
selves who  having  endeavored  to  prove  their  validity 
began  to  find  them  inadequate  to  the  requirements  of 
expanding  knowledge.  The  question  was  asked,  "  What 
is  the  origin  of  the  fittest?"  Given  the  fittest,  we  easily 
understand  how  it  is  perpetuated,  but  how  does  it  arise? 
Can  the  specific  beginnings  be  found  in  the  principle 
of  natural  selection?  It  seems  rather  curious  that  Darwin's 
own  answer  to  this  question  seems  to  have  been  overlooked, 
for  he  expressly  states  that  it  is  to  be  found  in  the  natural 
small  variations  that  obtain  among  species,  which,  if  use- 
ful, will  tend  to  be  preserved,  if  useless  to  be  extinguished. 

Gradually  the  ranks  broke  and  scientists  of  distinction — 
von  Baer,  von  Kolliker,  Virchow,  Nageli,  Wigand,  Hart- 
mann,  von  Sachs,  Eimer,  Delage,  Haacke,  Kassowitz, 
Cope,  Haberlandt,  Goethe,  Wolff,  Driesch,  Packard, 
Morgan,  Jaeckel,  Steinmann,  Korchinsky,  and  De  Vries 
— broke  away  declaring  that  the  origin  of  species  was  not 
to  be  found  in  Darwinism,  and  returned  to  the  teachings 
of  Lamarck,  that  inherited  acquired  characters  formed 
the  inception  of  the  specific  differences  (Neo-Lamarckism). 

This  must  not  be  interpreted,  however,  to  mean  that 
Darwinism  was  dead.  Indeed  there  was  soon  a  Neo- 
Darwinism  revival  with  a  goodly  following,  at  the  head 
of  which  stood  Weismann. 

Weismann's  doctrine  of  the  "inviolability  of  the 
germplasm"  as  first  expressed  appeared  to  be  opposed 
to  Darwin,  for  it  argued  that  nothing  could  appear 
,in  the  offspring  that  was  not  already  present  in  the 
germ  plasm,  hence  no  condition  to  which  an  organism 
was  subjected  could  modify  its  descendants,  see- 
ing that  the  germ  plasm  from  which  they  were  to 


296  BIOLOGY:  GENERAL  AND  MEDICAL 

descend  was  already  in  being.  But,  as  has  already 
been  shown  in  discussing  the  theory  in  the  chapter 
dealing  with  the  problems  of  inheritance,  Weismann 
was  later  obliged  to  modify  the  theory  and  to  admit 
that  the  germ  plasm  can  and  does  become  modified 
through  residence  in  its  host.  Such  a  conclusion  was 
inevitable;  amphimixis,  or  the  commingling  of  different 
germ  plasms,  could  never  account  for  divergence,  seeing 
that  originally  the  germ  plasm  was  all  the  same.  If  the 
germ  plasm  is  susceptible  of  modification,  as  Weismann 
himself  admits  and  we  must  conclude,  such  modifica- 
tions are  undoubtedly  governed  by  forces  acting  upon 
the  germ  plasm  while  in  the  host,  and  hence  probably  by 
conditions  to  which  the  host  is  subjected.  This  is  per- 
fectly in  accord  with  Darwin  and  made  Weismann  one 
of  the  strongest  of  the  Neo-Darwinians. 

The  Neo-Lamarckians,  however,  including  Herbert 
Spencer,  Packard,  Osborn,  Eimer,  among  their  early 
champions,  carried  on  a  bitter  warfare,  and  many  inter- 
esting phases  of  the  subject  were  discussed  during  1893 
and  1894  in  papers  by  Herbert  Spencer  on  the  one  side 
and  Weismann  on  the  other. 

The  question  at  issue  has  never  been  settled.  There 
are  present-day  scientists  who  see  no  reason  why  natural 
selection  may  not  account  for  the  origin  of  species,  there 
are  others  to  whom  it  is  totally  inadequate.  Indeed, 
one  scientist,  Korschinsky,  takes  a  diametrically  opposed 
view  and  appears  as  the  most  radical  anti-Darwinian, 
with  the  following  expressions  regarding  natural  selection: 

"The  origin  of  new  forms  can  only  occur  under  conditions 
favorable  to  them,  and  the  more  favorable  such  conditions  are, 
that  is,  the  less  severe  the  struggle  for  existence  is,  the  more 
energetic  is  their  development.  Under  severe  external  conditions 
new  forms  do  not  arise,  or  if  they  appear  they  are  extinguished. 

"The  struggle  for  existence,  and  the  selection  which  goes  hand 
in  hand  with  it,  compose  a  factor  which  restricts  new  appearing 
forms  and  restrains  wider  variations,  and  which  is  in  no  way 
favorable  to  the  production  of  new  forms.  It  is,  indeed,  an  inimical 
factor  in  evolution. 

"  Were  there  no  struggle  for  existence,  then  there  would  be  no 
extinguishing  of  arising  or  already  new  forms.  The  organic  world 


DIVERGENCE  297 

could  then  develop  into  a  mighty  tree,  whose  branches  could  all 
remain  in  blooming  condition,  so  that  the  now  isolated  extremest 
species  would  be  united  with  all  others  through  gradatory  forms. 

"The  adaptation  resulting  from  the  effects  of  the  struggle  for 
existence  is  absolutely  not  identical  with  advance  for  higher 
standing;  more  complex  forms  are  by  no  means  always  better 
adapted  to  outward  conditions  than  the  lower  ones.  The  evolution 
[here  used  by  the  author  as  synonymous  with  advance  or  pro- 
gressive complexity]  of  organisms  cannot  be  explained  in  a 
purely  mechanical  way.  In  order  to  explain  the  origin  of  higher 
forms  from  lower  forms  it  is  necessary  to  postulate  in  the  organ- 
isms a  special  tendency  to  advance  which  is  nearly  related  to,  or 
identical  with  the  tendency  to  vary,  which  tendency  compels  the 
organisms  to  advance  so  far  as  the  outward  conditions  permit." 

It  may  be  wise  to  add  that  such  views  have  not  met 
with  acceptance  even  among  the  most  urgent  anti- 
Darwinians. 

In  endeavoring  to  account  for  the  origin  of  species 
otherwise  than  by  natural  selection,  the  theory  of  "Muta- 
tion" has  taken  a  strong  hold  on  the  mind  of  the  day,  and 
has  found  a  champion  in  Hugo  De  Vries,  a  strong  anti- 
Darwinian.  This  Belgian  botanist  had  the  good  fortune  to 
discover  a  plant,  CEnothera  lamarkiana,  a  variety  of  prim- 
rose, at  a  time  when  it  appeared  to  undergo  a  sudden  trans- 
formation, diverging  from  its  customary  type  to  a  larger 
and  quite  different  one.  The  new  form,  which  differed 
sufficiently  to  constitute  a  new  variety,  if  not  a  new 
species,  appeared  to  undergo  the  change  "spontaneously," 
i.e.,  without  any  accountable  cause.  It  was,  in  other 
words,  a  " sport"  of  Nature.  The  new  individual,  how- 
ever, bred  true,  without  any  tendency  to  revert  to  its 
original  type,  and  has  remained  true.  This  has  led  De 
Vries  to  the  conclusion  that  new  species  arise  spontane- 
ously as  "freaks"  or  "sports,"  and  that  such  new  forms 
appear  infrequently  in  the  history  of  every  species  and 
serve  as  points  of  departure  from  the  old  type.  Accord- 
ing to  this  theory,  which  seems  to  be  widely  accepted 
,by  scientists  of  the  present  day,  species  arise  suddenly, 
by  mutation,  and  not  gradually  by  natural  selection. 
De  Vries  showed  quite  clearly  that  there  was  a  distinct 
difference  between  the  fluctuating  and  non-heritable 


298  BIOLOGY:  GENERAL  AND  MEDICAL 

variations  that  are  but  varieties  and  the  permanent 
and  heritable  variations  to  which  he  applies  the  term 
mutation. 

It  should  be  said,  however,  that  examples  of  such 
sudden  mutation  are  very  infrequent,  and  that  some 
of  them  were  known  to  Darwin,  as,  for  example,  the 
Ancon  sheep. 

The  scientists  of  to-day  are  fully  in  accord  that  all  the 
living  things  we  know  have  become  diversified  by  evolu- 
tion from  antecedent  and  simpler  forms,  and  these  from 
antecedent  simpler  forms  until  we  are  eventually  brought 
back  to  the  primordial  protoplasm.  There  is  no  contro- 
versy as  to  what  has  taken  place,  the  question  at  issue  is 
how  it  has  come  about.  The  further  back  we  go,  the 
more  difficult  the  question  becomes.  We  cannot 
imagine  the  nature  of  the  first  distinctly  living  organisms. 
As  they  must  have  been  of  very  soft  substance  and  de- 
rived their  support  from  substances  of  inorganic  nature 
uniformly  diffused  through  some  fluid  medium,  we  are 
probably  correct  in  supposing  that  they  were  aquatic 
and  marine.  Through  what  forces  this  elementary  sub- 
stance began  its  primary  differentiations  is  not  known. 
It  would  at  first  glance  seem  to  be  removed  from  every 
outside  influence  and,  therefore,  unlikely  to  be  either 
modifiable  or  modified;  but  on  second  thought  one 
remembers  that  the  ocean  is  not  uniform  in  its  conditions. 
It  reaches  to  the  poles  where  it  is  cold,  it  crosses  the 
equator  where  it  is  warm;  through  it  streams  of  warmer 
or  colder  water  flow;  into  it  rivers  of  fresh  water  empty; 
in  it  various  substances  dissolve;  its  shores,  which  form  a 
solid  sub-stratum,  are  washed  by  breakers;  its  surface 
is  touched  by  the  sun,  its  depths  are  in  perpetual  dark- 
ness. Surely,  under  these  diversified  conditions  living 
substance  if  modifiable  would  find  conditions  appropriate 
for  modification.  Indefinite  time  must  have  elapsed 
before  the  first  step  was  taken,  great  periods  of  time 
must  have  elapsed  before  differences  became  pronounced; 
but  once  started,  the  process  of  differentiation  and  diver- 


DIVERGENCE  299 

sification  progressed  in  the  sea  and  later  on  the  land 
until  the  world  of  to-day  arrived. 

The  vastness  of  time  necessary  for  the  evolutionary 
phenomena  was  at  first  supposed  to  be  one  of  the  strong- 
est arguments  against  it,  for  the  astronomers  and  geolo- 
gists found  it  impossible  to  admit  the  age  of  the  world's 
crust  to  be  sufficient  to  permit  them.  But  this  objection 
has  been  effaced,  for  the  discovery  of  radium  by  which 
the  sun  makes  good  its  heat  loss  and  the  further  dis- 
covery that  the  earth  possesses  self-sustaining  heat 
centres  and  is  not  entirely  dependent  upon  the  sun  for 
its  supply,  have  upset  all  past  calculations  in  cosmogonies 
and  now  permit  the  biologists  almost  infinite  time  for 
evolution. 

But  granting  the  process  of  evolution  in  progress,  by 
what  means  does  it  continue?  By  a  succession  of  fits 
and  starts,  leaps  and  bounds,  or  by  a  series  of  continuous 
and  almost  imperceptible  changes,  or  does  it  do  so  by 
means  of  both?  Present  opinion  is  in  favor  of  the  former; 
past  opinion  of  the  latter;  both  may  be  correct. 

REFERENCES. 

CHARLES  DARWIN:  "The  Origin  of  Species  by  Means  of  Natural 
Selection."  "The  Descent  of  Man."  "The  Variation 
of  Animals  and  Plants  under  Domestication." 

HERBERT  SPENCER:     "First  Principles."     "Principles  of  Biology." 

E.  D.  COPE:  "The  Origin  of  the  Fittest,"  N.  Y.,  1887.  "The 
Primary  Factors  in  Organic  Evolution,"  Chicago,  1896. 
"The  Origin  of  Genera,"  1868. 

GEORGE  JOHN  ROMANES:  "Darwin  And  After  Darwin,"  Chicago, 
1896. 

ERNST  HAECKEL:  "The  Riddle  of  the  Universe,"  Translated  by 
Joseph  McCabe,  N.  Y.,  1900.  "The  History  of  Creation." 

THOMAS   H.   HUXLEY:     "Man's    Place    in    Nature,"   and   other 


ROBERT  CHAMBERS:  "Vestiges  of  the  Natural  History  of  Crea- 
tion," London,  1887. 

HUGO  DE  VRIES:  "Species  and  Varieties,  their  Origin  by 
Mutation,"  Chicago,  1906. 

VERNON  S.  KELLOGG:     "Darwinism  To-day,"  N.  Y.,  1908. 

F.  W.  HUTTON:     "  Darwinism  and  Lamarckism."  N.  Y.,  1899. 


300  BIOLOGY:  GENERAL  AND  MEDICAL 

W.  H.  CONN:     "The  Method  of  Evolution,"  N.  Y.,  1900. 

JEAN  BAPTISTE  PIERRE  ANTOINE  DE  MONET  DE  LAMARCK:   "Ani- 

maux  sans  vertebres,"  1815.     "Philosophic  Zoologique," 

1809. 
ERASMUS  DARWIN:     "The  Botanic  Garden,"  1789.     "Zoonomia," 

1794-6. 
ALFRED   RUSSEL   WALLACE:     "Contributions  to  the  Theory  of 

Natural  Selection,"  New  York  and  London,  1870. 
THOMAS    ROBERT   MALTHUS:     "An    Essay   on   the   Principle    of 

Population,  etc.,"  London,  1826. 
T.  H.  MORGAN:     "Evolution  and  Adaptation,"  New  York,  1903. 


CHAPTER  XII. 
STRUCTURAL  RELATIONSHIP. 

In  the  earliest  Hebrew  Scriptures  we  find  living 
things  already  separated,  in  the  minds  of  the  writers, 
into  such  general  classes  as  "  grass, "  "herb  yielding  seed," 
"fruit  trees  yielding  fruit  after  their  kind,  whose  seed  is 
in  itself,"  "creeping  things,"  "fish," '"fowls  of  the  air," 
"beasts  of  the  field,"  and  "man,"  which  enabled  them 
to  be  collectively  mentioned,  and  paved  the  way  for 
future  more  precise  groupings,  To  these  writers,  how- 
ever, each  kind  was  separately  created  and  independent 
of  all  others. 

Grecian  philosophical  speculation  concerning  the 
origin  of  things  found  no  satisfaction  in  the  creation 
hypothesis,  and  at  an  early  date  the  idea  prevailed  that  the 
earth  and  its  creatures  arose  in  a  more  or  less  orderly  se- 
quence by  process  of  evolution.  With  as  much  thorough- 
ness as  their  familiarity  with  the  living  creatures  per- 
mitted, they  divided  them  into  groups  suggesting  the 
order  of  descent,  beginning  with  the  most  simple  and 
ending  with  the  most  complex.  These  endeavors  were, 
however,  much  impeded  by  superstitions  regarding  the 
ready  spontaneous  generation  of  almost  any  living  thing, 
and  the  equally  prevalent  belief  that  living  things  of 
one  kind  readily  metamorphosed  into  others. 

The  history  of  scientific  classification  seems  to  begin 
with  Aristotle,  who  as  an  anatomist  and  physiologist 
acquired  a  broad  knowledge  of  the  lower  animals  and 
divided  them  as  follows: 

Bloodless  Animals. — Insects 
Molluscs 
Crustacea 
Testacea 
301 


302  BIOLOGY:  GENERAL  AND  MEDICAL 

Blooded  Animals. — Fishes 

Amphibia 
Birds 
Mammals 

Here  the  matter  rested  for  centuries  without  important 
addition  or  alteration,  for  religion  displaced  science  and 
philosophy,  which  were  almost  forgotten  until  the 
Renaissance. 

The  next  important  classification  comes  to  us  from 
Linnseus,  the  father  of  botany  and  a  profound  thinker 
and  reasoner.  Not  a  versatile  zoologist,  but  one  to 
whom  zoology  owes  much  in  the  introduction  of  the 
binomial  nomenclature  and  in  the  description  of  all  the 
common  forms  of  animals,  he  gives  us  the  following 
very  simple  arrangement: 

I.  Mammalia 
II.  Aves 

III.  Amphibia 

IV.  Pisces 
V.  Insects 

VI.    Vermes 

Linnaeus  thus  departs  from  Aristotle  by  dispensing  with 
the  two  primary  divisions  of  Bloodless  and  Blooded 
animals. 

Aristotle  based  the  primary  groupings  upon  the  pres- 
ence or  absence  of  (red)  blood,  but  we  find  that  Lin- 
naeus abandoned  this  feature.  This  leads  us  to  inquire 
what  are  the  legitimate  characters  upon  which  classifi- 
cation can  be  based. 

Such  characters  are  purely  arbitrary  and  at  the  option 
of  the  systematist  by  whom  the  classifying  is  done. 
But  in  the  arbitrary  selection  of  the  characters  used  for 
the  purpose  one  thing  is  essential,  viz.,  that  they  be 
constant.  They  need  not  bear  any  reference  to  struc- 
tural or  functional  importance,  but  they  must  be  invari- 
able. Now,  when  we  scrutinize  Aristotle's  employment 
of  the  presence  or  absence  of  blood  as  a  primary  differ- 
ential character,  we  find  it  to  be  inconstant.  Blood  was, 


STRUCTURAL    RELATIONSHIP  303 

to  Aristotle,  a  red  fluid  that  escaped  from  an  organism 
when  injured.  That  blood  owed  its  redness  to  corpuscles, 
and  that  their  color  in  turn  depended  upon  the  quantity 
of  hemoglobin  they  contained  were  facts  beyond  his 
power  of  finding  out.  He  must  have  confused  any 
other  red  fluid  with  blood  had  he  found  it.  So  soon  as 
it  was  discovered  that  the  redness  of  the  blood  depended 
upon  the  hemoglobin  and  that  this  substance  appeared 
regularly  and  in  large  quantities  in  some  of  his  lowest 
groups,  the  differential  value  of  the  character  was  lost. 
What  is  true  of  Aristotle's  may  be  true  of  any  differential 
factor  in  classification — with  expanding  knowledge 
its  validity  may  be  destroyed.  The  problem  of  the 
systematist  is  to  find  the  invariable  characters  and 
build  upon  them. 

The  purpose  of  classification  may  differ,  and  therefore 
the  means  may  also  differ.  The  earliest  classifications 
were  supposed  to  partake  of  the  nature  of  a  family  tree 
and  were  based  upon  the  supposition  that  the  higher 
forms  of  existing  animals  arose  from  the  lower  forms  by 
some  more  or  less  direct  mutation. 

Ancient  observers  had  not  overlooked  the  fact  that 
bones  and  teeth  of  extraordinary  size  and  remarkable 
shape  were  found  here  and  there  upon  the  earth's  sur- 
face, and  that  beds  of  marine  shells  were  sometimes  to 
be  found  upon  the  uplands  and  in  the  mountains. 
These  puzzling  circumstances  were  generally  accounted 
for  upon  the  supposition  that  in  the  prehistoric  times 
giants  and  chimerical  monsters  had  inhabited  the  earth, 
and  that  there  had  been  great  deluges  when  the  sea 
arose  and  covered  the  highest  mountain  tops.  Myths 
to  account  for  such  things  are  to  be  found  among  all 
nations.  Later  scientific  scrutiny  showed  that  these 
"fossil"  remains  for  the  most  part  resemble  living 
.creatures,  though  some  are  dissimilar.  Gradually 
Geology,  the  study  of  the  formation  of  the  earth's  crust, 
and  Paleontology,  the  study  of  extinct  forms  of  life, 
developed  as  special  departments  of  investigation,  receiv- 


304  BIOLOGY:  GENERAL  AND  MEDICAL 

ing  great  progress  through  the  labors  of  Lamarck,  who 
founded  the  science  of  invertebrate  paleontology,  and 
Cuvier  who  founded  that  of  vertebrate  paleontology, 
and  it  was  discovered  that  the  "fossil  remains"  afforded 
an  insight  into  the  nature  and  structure  of  creatures 
that  had  long  ago  inhabited  the  earth,  but  became  dis- 
placed by  now  existing  forms.  Fossil  animals,  there- 
fore, came  to  require  a  place  in  the  genealogical  tree,  or 
system  of  classification. 

Should  it  ever  become  possible  to  become  acquainted 
with  all  of  the  animals  that  have  lived  as  well  as  all  of 
those  that  now  live,  and  to  place  them  in  correct  and 
orderly  position  with  reference  to  one  another,  the  ar- 
rangement would  show  the  exact  genealogy  of  every 
group  and  its  complete  family  tree. 

Such  a  family  tree  would  also  show  that  the  different 
groups  of  animals  that  we  now  know  have  not,  as  the 
early  systematists  imagined,  developed  one  into  the  other, 
but  that  they  are  simply  branches  of  the  same  great 
family  tree  so  that  to  get  from  one  to  the  other  one 
would  be  obliged  to  descend  one  branch  to  some  common 
intermediate,  or  even  go  back  to  the  main  trunk  and 
ascend  again  in  order  to  reach  the  new  branch.  This 
is  a  most  fundamental  conception.  The  various  animals 
we  now  know  are  the  newest  buds  and  sprouts  upon  the 
apical  twigs  of  boughs,  branches,  and  limbs  of  the  tree 
of  life  that  has  been  growing  and  spreading  ever  since 
life  first  made  its  appearance. 

The  ambition  of  every  systematist  of  the  present  time 
is  to  so  arrange  the  known  living  and  extinct  organisms 
as  to  make  them  find  their  proper  places  in  the  evolution- 
ary sequence.  This  is  quite  a  different  matter  from  that 
of  arranging  them  in  the  order  of  development  one  into 
the  other. 

How  nearly  we  are  in  a  position  to  complete  the  gene- 
alogical tree  may  be  judged  by  a  hasty  comparison  of 
the  known  living  and  fossil  forms.  Allowing  a  liberal 
margin  for  error,  it  may  be  surmised  that  there  are  known 


STRUCTURAL    RELATIONSHIP 


305 


T&rds 

70000 


Reptiles 

3000 


Amphibians 

100 


Protozoa 

4000 

PIG.   107. — Diagram  showing  the  general  relations  of  the  chief  divisions  of 
the  animal  kingdom.     The  number  of  species  belonging  to  each  is  roughly 
approximate  only.     (Galloway?) 
20 


306  BIOLOGY:  GENERAL  AND  MEDICAL 

at  the  present  time  not  less  than  500,000  different  species 
of  living  animals  and  about  75,000  different  species  of 
fossil  animals.  Reflection  upon  the  conditions  of  evolu- 
tion considered  in  another  chapter  should  convince  one 
that  the  number  of  species  now  living  must  be  infini- 
tesimal compared  with  the  great  number  of  extinct  forms 
from  which  they  have  descended  and  to  which  they  are 
related.  In  consequence  the  genealogical  tree  is  deficient 
in  many  of  its  parts  which  can  be  linked  together  only  in 
imagination.  Some  of  these  gaps  can,  and  no  doubt 
will,  be  filled  in  time  by  the  discovery  of  additional  fossil 
species,  but  many  of  them,  embracing  soft-bodied  ani- 
mals that  leave  no  relics  behind  them,  can  never  be 
filled  in. 

An  important  improvement  in  classification  came 
from  the  great  French  naturalist,  Cuvier  (1798).  The 
chief  divisions  are  as  follows: 

Branch  I. — Animalia  Vertebrata: 
Class  1.  Mammalia 

2.  Aves 

3.  Reptilia 

4.  Pisces 
Branch  II. — Animalia  Mollusca: 

Class  1.  Cephalopoda 

2.  Pteropoda 

3.  Gasteropoda 

4.  Acephala 

5.  Brachiopoda 

6.  Cirrhopoda 
Branch  III. — Animalia  Articulata: 

Class  1.  Annelides 

2.  Crustacea 

3.  Arachnides 

4.  Insects 
Branch  IV. — Animalia  Radiata: 

Class  1.  Echinoderms 

2.  Intestinal  worms 


STRUCTURAL   RELATIONSHIP  307 

3.  Acalephae 

4.  Polypi 

5.  Infusoria 

In  1801,  Lamarck  introduced  a  new  term — "Inverte- 
brata" — and  a  new  idea,  that  of  physiology,  into  classifi- 
cation; he  also  made  a  revision  of  Cuvier's  classification, 
leaving  the  vertebrates  unchanged. 

Invertebrata. 

I.  Insensitive  Animals — Class  1.  Infusoria 

2.  Polypi 

3.  Radiaria 

4.  Tunicata 

5.  Vermes 

II.  Sensitive  Animals — Class     6.  Insects 

7.  Arachnids 

8.  Crustacea 

9.  Annelids 

10.  Cirripeds 

11.  Conchifera 

12.  Mollusks 
Vertebrata.— Class  13.  Pisces 

14.  Reptilia 

15.  Aves 

16.  Mammals 

The  employment  of  function  as  the  correlative  of  struc- 
ture in  classifying  animals  was  overdone  by  Oken,  who, 
in  1810,  published  the  following  classification  of  the 
Invertebrates: 

Grade  I. — Intestinal  Animals. 

Cycle  I.  Digestive  Animals — Radiata 

Class  1.  Infusoria  (stomach  animals) 

2.  Polypi  (intestine  animals) 

3.  Acalephae  (lacteal  animals) 
Cycle  II.  Circulative  Animals — 

Class  4.  Acephala  (biauriculate  animals) 

5.  Gasteropoda    (uniauriculate   ani- 
mals) 

6.  Cephalopoda    (bicardial  animals) 


308  BIOLOGY:  GENERAL  AND  MEDICAL 

Cycle  III.     Respirative  animals — 

7.  Worms  (skin  animals) 

8.  Crustacea    (branchial    animals) 

9.  Insects  (tracheal  animals) 

A  new  idea  followed  the  active  pursuit  of  embryology 
by  the  German  naturalists  and  is  expressed  by  von 
Baer,  whose  idea  that  "ontogeny  recapitulates  phy- 
logeny"  led  him  to  propose  the  following  classification 
based  upon  the  embryological  development  of  the  mem- 
bers of  the  various  groups: 

I.  Peripheral  type  (Radiata) 

II.  Massive  type  (Mollusca) 

III.  Longitudinal  type  (Articulata) 

I V.  Doubly  symmetrical  type  ( Vertebrata) 

"Thus,  von  Baer,  with  his  classification,  based  on 
embryological  principles,  and  Cuvier,  with  his,  founded 
on  comparative  anatomy,  arrived  at  very  similar  con- 
clusions, viz. :  that  animals  are  built  upon  four  general 
plans  and  fall  into  four  general  groups.  In  the  end,  the 
system  of  Cuvier  triumphed  over  that  of  the  natural 
philosophers." 

Revisions  of  the  classification  of  the  invertebrata 
were  published  by  Agassiz  and  later  by  Huxley  and  did 
much  to  assist  in  promoting  research  upon  the  animals 
of  these  divisions.  As,  however,  their  classifications 
were  not  based  upon  any  essentially  new  idea,  it  seems 
proper  to  pass  on  to  the  modern  classification. 

According  to  the  plan  adopted  at  present,  the  whole 
animal  kingdom  is  divided  into  two  Siib-divisions, 
the  Protozoa  and  the  Metazoa.  Each  sub-division  is 
made  up  of  certain  grand  groups  or  Phyla  which  repre- 
sent more  or  less  well-marked  plans  of  structure,  and 
form  the  points  about  which  all  the  animals  constructed 
upon  a  similar  general  plan  are  arranged.  Each  phylum 
includes  a  number  of  Classes,  each  of  which  is  composed 
of  organisms  which,  though  phyletically  related,  differ 
in  some  constant  feature,  such  as  having  six  legs  or 


STRUCTURAL   RELATIONSHIP  309 

eight  legs,  etc.  Each  class,  in  turn,  is  composed  of 
Orders  into  which  closely  related  Families  fall,  and  each 
family  is  composed  of  Genera,  which  in  turn  embrace  the 
smallest  groups  or  Species. 

Each  group  is  arbitrary,  but  the  characters  upon  which 
the  larger  groups  are  founded  have  been  shown  by  ex- 
perience to  be  so  constant  as  to  permit  of  little  present 
modification.  The  changing  groups  are  the  families, 
genera,  and  species,  and  of  these  the  genera  and  species 
are  subject  to  the  greatest  mobility.  There  is  no  fixed 
opinion  as  to  what  shall  constitute  a  species  or  what  shall 
be  called  a  variety  of  a  species.  In  groups  of  organisms 
much  studied  specific  differences  are  so  minute  that  only 
the  most  careful  scrutiny  with  the  microscope  can  dis- 
cover them;  in  groups  little  studied  the  species  differ 
quite  as  widely  as  the  genera  of  much  studied  groups. 

Cuvier  tried  to  make  the  criterion  of  specific  dif- 
ferentiation the  tendency  of  the  organism  to  breed  true, 
but  as  the  breeding  of  vast  numbers  of  organisms  is 
something  concerning  which  no  information  is  available 
and  upon  which  none  may  be  attainable,  it  becomes 
impossible  to  follow  the  suggestion. 

According  to  the  general  acceptation,  a  species  is 
composed  of  individual  organisms  whose  dissimilarities 
are  so  slight  and  inconstant  as  not  to  be  definite. 
Many  think  that  specific  differences  should  be  struc- 
tural only;  others  admit  differences  of  coloration  as  marks 
of  specific  differentiation.  Those  who  base  specific 
characters  upon  structural  differences  alone,  separate 
similarly  constructed  but  differently  colored  or  different- 
sized  organisms  into  still  lower  groups  known  as  varieties. 
Varieties,  however,  may  differ  among  themselves  in 
structure  as  species  sometimes  do.  Thus,  among  the 
barnyard  fowls  the  rose  comb  and  the  toothed  comb 
and  the  presence  or  absence  of  spurs  are  well-marked 
structural  differences,  yet  are  not  looked  upon  as  specific, 
and  no  one  can  gainsay  that  there  are  structural  dif- 
ferences between  the  bull-dog,  the  greyhound,  and  the 


310  BIOLOGY:  GENERAL  AND  MEDICAL 

dachschund,  though  all  dogs  are  regarded  as  of  the 
same  species. 

Specific  grouping  is,  therefore,  chiefly  a  matter  of 
personal  equation  with  the  systematist.  Fortunately, 
it  has  little  or  no  practical  importance  in  the  general 
plan  of  classification. 

At  this  point  it  may  be  well  to  digress  and  say  a  few 
words  about  the  binomial  nomenclature  of  Linnaeus, 
which  has  now  been  adopted  by  international  agree- 
ment among  scientific  men.  Experience  has  shown 
that  in  naming  men  but  small  confusion  occurs  when 
each  individual  receives  two  names.  Thus  John  Smith 
is  understood  to  be  a  member  of  the  Smith  family  indi- 
vidually known  as  John.  As,  however,  the  Smith  family 
is  large,  and  John  is  a  favorite  name,  there  may  be  several 
individuals  known  as  John  Smith.  Such  confusion 
rarely  arises,  however,  in  scientific  nomenclature,  be- 
cause the  name  is  not  the  designation  of  an  individual, 
but  of  a  kind.  Greek  and  Latin  names  have  been 
agreed  upon  for  scientific  nomenclature,  because  they 
are,  so  to  speak,  international  languages  and  less  likely 
to  cause  confusion  than  English,  German,  French,  or 
Italian.  For  example,  the  common  black  and  white 
hornet  that  builds  the  large  rounded  paper  nests 
was  described  by  Linnsus  as  Vespa  maculata.  By 
agreement  the  name  of  every  organism  is  followed  by 
the  name  (usually  abbreviated)  of  the  man  first  describ- 
ing it.  The  name  of  this  insect  is,  therefore,  now  written 
Vespa  maculata  Linn.  The  word  maculata  is  the  name 
of  the  species  or  particular  kind,  and  is  always  written 
with  a  small  letter,  even  if  derived  from  a  proper  noun. 
Thus  Megatherium  cuveri,  Mimisa  cressoni,  Bacillus 
welchi,  etc.  This  specific  name  must  be  the  first  name 
applied,  no  matter  by  whom  or  when,  and  regardless  of 
its  appropriateness.  The  specific  name  is  always 
Latin  or  latinized.  The  specific  name  by  itself  is  com- 
paratively meaningless,  just  as  though  one  spoke  of 
Clarence.  Who  is  Clarence?  What  Clarence?  So, 


STRUCTURAL   RELATIONSHIP  311 

should  one  speak  of  "  macidata,"  it  would  mean  nothing, 
since  many  species,  because  of  their  spotted  character, 
might  have  received  that  name.  When,  however,  one 
says  Vespa  maculata  or  Amblystoma  maculata,  it  at  once 
becomes  quite  clear  what  animal  is  intended.  The 
generic  name,  which  comes  first,  may  be  Latin,  and  is  of 
all  the  older  genera,  but  is  now,  by  preference,  of  Greek 
derivation.  The  specific  name  may  never  change,  but 
the  generic  name  may  have  to  be  changed  from  time  to 
time  because  many  genera  when  carefully  studied  are 
found  to  be  divisible  into  several  new  genera.  When 
this  happens,  it  is  agreed  that  new  generic  names  may 
be  used,  but  that  the  old  generic  name  if  not  continued 
for  one  of  the  newly  formed  divisions  may  never  be 
employed  again. 

The  following  outlines  of  modern  classification  are 
introduced  in  order  that  readers  not  familiar  with  botany 
or  zoology  may  secure  an  approximately  correct  idea 
of  .the  general  relations  that  living  things  bear  to  one 
another. 

THE  PLANT  KINGDOM. 

Phylum  I. 

Thallophyta.     The  thallus  plants. 

Series  1. — Algae,  unicellular  forms,  pond  weeds, 

sea  weeds,  etc. 
Class         I. — Cyanophyceae,  the  blue-green 


Class       II. — Chlorophycese,  the  green  algae. 
Class     III. — Phacophycese,      the      brown 


Class     IV. — Rhodophycese,  the  red  algae. 
Series  2. — Fungi,     bacteria,     yeasts,     moulds, 

smuts,  mushrooms,  etc. 
Class        V. — Schizomycetes,  the  bacteria. 
Class      VI. — Saccharomycetes,  the  yeasts. 
Class     VII. — Phycomycetes,   the  alga-like 

fungi. 
Class  VIII. — Ascomycetes,  the  sac-fungi. 


312  BIOLOGY:  GENERAL  AND  MEDICAL 

Class     IX. — Basidiomycetes,   the  basidia 

fungi. 
Phylum  II. 

Bryophyta. — The  moss-like  plants. 
Class  1. — Hepaticae,  the  liverworts. 
Class  2. — Musci,  the  mosses. 
Phyhim  III. 

Pteridophyta.— The  ferns  and  their  allies. 
Class  1. — Filicineae,  true  ferns. 
Class  2. — Equistineae,  the  horse  tails. 
Class  3. — Lycepodinese,  the  club  mosses. 
Phylum  IV. 

Spermatophyta. — The  seed-bearing  plants. 
Sub-division  1. — Gymnospermse. 
Sub-division  2. — Angiospermse. 
Class    I. — Monocotyledons. 
Class  II. — Dicotyledons. 

While  most  of  the  groups  in  this  classification  include 
forms  naturally  related,  the  algae  and  fungi,  respectively, 
include  sub-groups  whose  relationship  is  doubtful. 
Thus,  the  bacteria  are  very  likely  more  nearly  related 
to  certain  of  the  algae  than  to  any  fungi,  though  in  habit 
of  life  they  are  like  the  latter. 

THE  ANIMAL   KINGDOM. 

Division  I.  Protozoa. — Unicellular  animals. 

Phylum — Protozoa.  —  Sub-phyla.  —  A.  Gymno- 
nomyxa  (without  perma- 
nent form). 

B.  Corticata  (with  perma- 
nent form). 

Class  I. — Rhizopoda. — Naked  protoplas- 
mic organisms  with  pseudo- 
podia,  without  a  localized  oral 
orifice.  Multiply  by  binary 
fission. 


STRUCTURAL   RELATIONSHIP  313 

Class  II. — Infusoria. — Unicellular  organ- 
isms provided  with  cilia  or 
flagella  and  having  a  localized 
oral  orifice.  Multiply  by  bi- 
nary fission. 

Class  III. — Sporozoa.  — Protoplasmic  or- 
ganism usually  naked  without 
an  oral  orifice.  Multiply  by 
dividing  into  many  small 
spores. 
Division  II. — Metazoa. — Multicellular  animals. 

A.  Without  true  ccelomic  cavity — Radially  symmet- 
rical. 

1.  Without  a  notochord — Invertebrata. 
Phylu  m — Porifera . 

The  sponges. — Characterized  by  many 
incurrent  openings  and  only  one  excur- 
rent  opening.  Axially  symmetric.  Sexu- 
ally reproductive. 

Class    I. — Calcarea. — Skeleton  of  cal- 
careous spicules. 

Class  II. — Non-calcarea. — Skeleton  of 
siliceous  spicules  and  horny 
fibres. 
Phylum — Ccelenterata. 

The  jelly-fishes. — Characterized  by  a  single 
mouth  which  also  functionates  as  an 
anus.  Possess  stinging  or  nettling 
cells. 

Class  I. — Hydrozoa. 
Class  II. — Scyphozoa. 
Class  III. — Actinozoa. 
Class  IV. — Ctenophora. 

B.  With  a  true  coelomic   cavity  —  Radially    symmet- 
rical. 

Phylum— Echinodermata. 

The  star-fishes,  sea-urchins,  sea-cucum- 
bers, etc. 


314  BIOLOGY:  GENERAL  AND  MEDICAL 

Class      I. — Asteroidea — star-fishes. 
Class    II. — Ophiuroidea — brittle   sea- 
stars. 
Class  III. —  Echinoidea  —  sea-urchins 

and  sand-dollars. 
Class  IV. — Crinoidea  — sea-lilies    and 

feather-stars. 

Class     V. —  Holothuroidea  —  sea -cu- 
cumbers. 

C.  With  a  true  ccelomic  cavity — bilaterally  symmet- 
rical. 

Phylum — Platyhelminthes. 

The  unsegmented  worms,  etc. 
Class      I. — Cestoda. 
Class    II.— Trematoda. 
Class  III.—  Turbellaria. 
Phylum — Nemathelminthes. 

Round    worms,     threadworms,  etc. 
Phylum — Molluscoidea. 
Mollusk-like  animals. 
Class    I. — Polyzoa. 
Class  II. — Brachiopoda. 
Phyl  u  m —  Trochelminthes. 

The  wheel  animalcules  or  rotifers. 
Phylum — Annulata. 

The  segmented  worms. 

Class      I. — Chsetopoda,  bristle-footed 

worms. 

Sub-class    A. — Polychseta    (with 
numerous  bristles). 
Sub-class    B. — Oligochseta  (with 
few  bristles) . 

Class    II. — Discophora — sucker-bear- 
ing worms. 

Class  III. — Archiannelida. 
Class  IV. — Siphunculoidae. 
Class  V.—  Chsetognatha. 


STRUCTURAL   RELATIONSHIP  315 

Phylum — Mollusca. 

Class      I. — Pelecypoda    or     Lamelli- 
branchiata  —  mussels,    oysters, 
and  other  bivalves. 
Class    II. — Gasteropoda — univalves  or 

shell-less  mollusks. 

Class  III. — Cephalopoda  —  squids, 
devil-fishes,  etc. 

Phylum — A  rthropoda. 
Jointed  animals. 

Class  I. — Crustacea  —  crabs,  lob- 
sters, cray-fish,  barnacles, 
etc. 

Class  II. — Onychophora — centipede- 
like. 

Class  III. — Myriapoda  —  centipedes, 
etc. 

Class  IV. — Hexapoda — insects. 

Class  V. — Arachnida  —  spiders, 
scorpions,  etc. 

2.  With  a  notochord. 
Phylum — Chlordata. 

Sub-phylum  I. — Atriozoa. 

Class    I. — Urochordata  (Tunicata). 
Class  II. — Cephalocordata    (A  m  p  h  i- 
oxus). 

Sub-phylum  II. — Vertebrata. 

Class     I. — Pisces — fishes. 

Class  II. — Amphibia  (Batrachia) — 
frogs,  toads,  salamanders. 

Class  III. — Reptilia — lizards,  croco- 
diles, alligators,  tortoises, 
snakes. 

Class  IV.— Aves— Birds. 

Class     V. — Mammalia — mammals. 


316  BIOLOGY:  GENERAL  AND  MEDICAL 

REFERENCES. 

"The   New   International   Encyclopedia,"    N.   Y.,   Dodd,  Meade 

&  Co.,  1907. 
A.     T.     MASTERMAN:     "Elementary    Text-book     of     Zoology," 

Edinburgh,  1902. 

T.  W.  GALLOWAY:     "First  Course  in  Zoology,"  Phila.,  1906. 
E.  STR  \SBURGER,  F.  NOLL,  H.  SCHENK,   and  G.  KARSTEN:     "A 

Text-book  of  Botany.     "Third  Engish  edition  by  W.  H. 

Lang,  N.  Y.,  1908. 


CHAPTER  XIII. 
BLOOD  RELATIONSHIP. 

It  may  justly  be  supposed  that,  as  living  things  di- 
verged morphologically,  they  also  diverged  physiologi- 
cally, and  indeed  this  fact  has  been  dwelt  upon  in 
various  relations.  Thus,  with  reference  to  nutrition,  we 
find  plants  agreeing  in  the  function  of  constructing  their 
protoplasm  out  of  inorganic  compounds,  and  animals 
diverging  from  them  in  the  loss  of  this  function.  Aquatic 
and  terrestrial  forms  of  life  differ  in  their  method  of 
absorbing  oxygen  and  in  the  quantity  essential  to  their 
metabolism.  Salt-  and  fresh-water  organisms  differ  in 
their  tolerance  to  sodium  chloride  and  other  salts.  Differ- 
ences in  diet,  in  function,  and  in  metabolism  continue  to 
increase,  until  we  find  it  not  infrequently  happening  that 
the  flesh  of  one  animal  is  poisonous  for  another,  and 
occasionally  happening  that  the  body  juice  of  one 
animal  is  poisonous  when  introduced  into  another. 

In  nearly  all  cases  the  body  juices  of  one  animal,  when 
introduced  into  the  blood  or  tissues  of  a  different  kind  of 
animal,  is  capable  of  acting  as  an  antigen,  i.e.,  of  exciting 
a  disturbance  in  the  form  of  a  chemico-physiological  "re- 
action." 

The  reactions  thus  induced  vary  according  to  the  nature 
of  the  experiment,  as  will  be  shown  in  the  chapter  upon 
Infection  and  Immunity.  When  the  antigens  are  adminis- 
tered in  small  doses,  frequently  repeated,  so  that  no  harm  is 
effected,  they  usually  induce  the  formation  of  self-dissolving, 
self-neutralizing,  self-agglutinating,  or  self-precipitating 
antibodies.  Zooprecipitins,  phytopredpitins,  hemolysins, 
cytotoxins,  and  antitoxins  are  examples  later  to  be  discussed. 

The  term  "blood-relationship"  has  been  introduced 
by  Nuttall  to  express  certain  physiologico-chemical 
resemblances  found  to  obtain  among  different  kinds  of 
317 


318  BIOLOGY:  GENERAL  AND  MEDICAL 

animals.  His  tests  for  determining  it  were  the  occurrence 
of  the  phenomena  of  specific  precipitation  and  hemolysis. 

The  studies  of  the  blood  leading  to  present  knowledge 
began  with  the  work  of  Creite,  who,  in  1869,  found  that 
heterologous  bloods  not  infrequently  caused  hemolysis  or  so- 
lution of  the  red  blood  corpuscles  of  the  animal  into  which 
the  blood  was  injected.  Landois,  in  1875,  found  that  the 
transfusion  of  heterologous  blood  into  an  animal  not  in- 
frequently caused  its  death.  Bordet,  in  1898,  found  that 
when  guinea-pigs  were  given  frequent  intraperitoneal  in- 
jections of  defibrinated  rabbits'  blood,  their  blood  serum 
acquired  the  property  of  dissolving  rabbits'  blood  cor- 
puscles in  vitro  (hemolysis);  Ehrlich  and  Morgenroth,  in 
1899,  showed  the  mechanism  of  such  blood  corpuscle  solu- 
tion. Tchistowich,  in  1899,  showed  that  eel's  serum  in- 
jected into  animals  produced  a  reaction  in  which  the  ani- 
mals acquired  immunity  to  its  poisonous  effects,  as  well  as 
their  serum  the  power  to  form  a  precipitate  when  added  to 
the  eel's  serum  in  vitro.  Uhlenhuth,  in  1900-1901,  found 
that  the  precipitation  resulting  from  the  addition  of  the 
immune  serum  to  the  antigen  by  whose  stimulation 
the  immune  character  of  the  serum  was  developed,  was  so 
specific  as  to  be  of  use  in  forensic  medicine  for  the  cer- 
tain differentiation  of  blood  stains.  Wassermann  and 
Schutze,  in  1900,  prepared  a  serum  by  injecting  rabbits 
with  human  blood,  and  tested  its  precipitating  proper- 
ties upon  twenty-three  different  kinds  of  blood,  finding 
the  precipitate  most  marked  the  nearer  the  animal  was 
related  to  man. 

The  work  of  Nuttall  and  his  associates,  "  Blood-immu- 
nity and  Blood-relationship, "  appeared  in  1904,  and 
dwells  exhaustively  upon  the  reaction  of  precipitation 
in  all  its  phyletic  relations. 

Through  a  study  of  the  specific  precipitins  we  learn 
that  though  the  reaction  is  specific  —  that  is,  takes 
place  in  greatest  quantity  and  with  greatest  rapidity 
when  the  antibody  is  permitted  to  act  upon  its  own 
antigen,  the  antigens  derived  from  closely  related  ani- 
mals and  plants  have  sufficient  chemical  or  physiologi- 


BLOOD    RELATIONSHIP 


319 


cal  resemblance    to     give    similar    qualitative    though 
different  quantitative  results. 

To  determine  this  we  proceed  as  follows:  a  rabbit  is 
given  intraperitoneal  injections  of  5-10  c.c.   of  defibri- 


FIG.    108. — Apparatus   for   quantitative   estimation   of   specific   precipitation 
(Nultall  and  Inchley,  in  "Journal  of  Hygiene.") 

nated  human  blood  twice  weekly  for  about  six  weeks,  then 
bled  about  a  week  after  the  last  injection,  and  the  clear 
serum  separated  from  the  clotted  blood.  We  thus  obtain 
a  reagent  which  when  added  to  clear  human  blood  serum 
immediately  gives  a  copious  white  precipitate.  If  the 
antiserum  be  diluted  1:50  or  1:100  as  a  standard,  so 
that  it  shall  always  be  present  in  uniform  quantity  in 
all  the  tests  made,  and  the  serums  to  be  tested  for  blood 
relationship  given  various  dilutions — 1:100,  1:1000 


320  BIOLOGY:  GENERAL  AND  MEDICAL 

1:10,000,  1:100,000,  etc.— and  the  mixtures  of  the  test 
antiserum  and  the  serum  to  be  tested  allowed  to  stand 
for,  say,  thirty  minutes  as  a  fixed  period,  it  will  be  found 
that  the  reaction  of  precipitation  is  specific  in  that  the 
homologous  bloods  precipitate  in  greater  dilution  and  in 
larger  quantity  than  heterologous  bloods.  Thus,  human 
serum  may  be  precipitated  with  anti-human  serum  in 
dilutions  even  reaching  1:100,000;  the  blood  of  anthro- 
poid apes  in  slightly  less  dilutions;  those  of  other  apes  in 
decidedly  less  dilutions;  those  of  lower  monkeys  in  less 
and  less  dilution  the  further  they  are  zoologically  removed 
from  man;  those  of  lower  mammals  only  in  the  concen- 
trated form,  if  at  all;  and  the  bloods  of  still  lower  verte- 
brates and  invertebrates  not  at  all. 

If,  instead  of  treating  the  rabbit  with  human  blood, 
injections  of  bovine  blood  be  used  and  anti-bovine 
serum  secured,  the  precipitation  measured  by  the  same 
method  is  found  to  occur  in  greatest  intensity  with  ox- 
blood,  then  with  the  bloods  of  other  bovida?,  then  with 
those  of  animals  closely  related  to  the  bovidae,  and  not 
at  all  with  those  of  animals  remote  from  the  bovidse. 

We  thus  have  a  physiologico-chemical  method  of  con- 
firming the  morphological  classification  of  animals. 
It  is  of  interest  to  know  that  the  one  bears  out  the 
other  in  practically  all  cases. 

The  general  principle  may  therefore  be  stated  as 
follows:  If  the  blood  or  body  juice  Of  one  organism 
is  capable  of  acting  as  an  antigen  when  introduced  into 
another  organism,  the  two  are  not  closely  related.  The 
index  of  antigenic  activity  is  to  be  found  in  the  quanti- 
tative reaction  of  precipitation  resulting  from  the  action 
of  the  antibody  upon  the  homologous  serum. 

A  most  interesting  and  important  recent  addition  to 
our  knowledge  of  blood  relationships,  thought  by  its 
authors  to  be  more  accurate  and  discriminative  than 
the  zooprecipitins,  is  the  crystallographic  character 
of  the  hemoglobin  of  the  blood.  This  is  described  in 
''The  Differentiation  and  Specificity  of  Corresponding 
Proteins  and  Other  Vital  Substances  in  Relation  to 


PLATE  2 


Oxyhemoglobin  from  a  fish  (carp).     Oxyhemoglobin  from  an  amphibian 

(necturus). 


Oxyhemoglobin    from    a    reptile     Oxyhemoglobin  from  a  bird  (goose), 
(python). 


\      ' 

/  I^f  **t£3i.*P 

Oxyhemoglobin   from    a    mammal     Oxyhemoglobin    from   a    mammal 
(dog).  (guinea-pig). 

Plate  showing  the  different  Oxyhemoglobin  crystals  from  different 
classes  of  vertebrates.  (From  photographs  kindly  given  the  author  by 
Prof.  Edward  T.  Reichert.) 


BLOOD   RELATIONSHIP  321 

Biological  Classification  and  Organic  Evolution:  The 
Crystallography  of  Hemoglobins, "  by  Edward  Tyson 
Reichert  and  Amos  Peaslee  Brown  (published  by  the 
Carnegie  Institution  of  Washington,  1909). 

The  following  extracts  give  the  deductions  from  an 
immense  amount  of  painstaking  and  difficult  experi- 
ment and  investigation: 

"The  authors  feel  that  "The  trend  of  modern  biological  science 
seems  to  be  irresistibly  toward  the  explanation  of  all  vital  phe- 
nomena on  a  physico-chemical  basis."  "The  striking  parallelisms 
that  have  been  shown  to  exist  in  the  properties  and  reactions  of 
colloidal  and  crystalloidal  matter  in  vitro  and  in  the  living  organ- 
ism lead  to  the  assumption  that  protoplasm  may  be  looked  upon 
as  consisting  of  an  extremely  complex  solution  of  interacting  and 
interpendent  colloids  and  crystalloids,  and  therefore  that  the 
phenomena  of  life  are  manifestations  of  colloidal  and  crystalloidal 
interactions  of  a  peculiarly  organized  solution.  We  imagine  this 
solution  to  consist  mainly  of  proteins  with  various  organic  and 
inorganic  substances.  The  constant  presence  of  protein,  fat, 
carbohydrate,  and  inorganic  salts,  together  with  the  existence  of 
protein-fat,  protein-carbohydrate,  and  protein-inorganic  salt 
combinations,  justifies  the  belief  that  not  only  such  substances, 
but  also  such  combinations  are  absolutely  essential  to  the  exist- 
ence of  life." 

"The  very  important  fact  that  the  physical,  nutritive,  or  toxic 
properties  of  given  substances  may  be  greatly  altered  by  a  very 
slight  change  in  the  arrangement  of  the  atoms  or  groups  of  mole- 
cules may  be  assumed  to  be  conclusive  evidence  that  a  trifling  modi- 
fication in  the  chemical  constitution  of  a  vital  substance  may  give 
rise  even  to  a  profound  alteration  in  its  physiological  properties." 

"This  coupled  with  the  fact  that  differences  in  centesimal  com- 
position have  proved  very  inadequate  to  explain  the  differences  in 
the  phenomena  of  living  matter,  implies  that  a  much  greater 
degree  of  importance  is  to  be  attached  to  peculiarities  of  chemical 
constitution  than  is  usually  recognized." 

"The  possibility  of  an  inconceivable  number  of  constitutional 
differences  in  any  given  protein  are  instanced  in  the  fact  that  the 
serum  albumin  molecule  may,  as  has  been  estimated,  have  as 
many  as  1,000  million  stereoisomers.  If  we  assume  that  serum 
globulin,  myoalbumin,  and  other  of  the  highest  proteins  may 
have  a  similar  number,  and  that  the  simpler  proteins  and  the  fats 
and  carbohydrates,  and  perhaps  other  complex  organic  substances, 
may  each  have  only  a  fraction  of  this  number,  it  can  readily  be 
conceived  how,  primarily  by  differences  in  chemical  constitution 
of  vital  substances  and  secondarily  by  differences  in  chemical 
composition,  there  might  be  brought  about  all  those  differences 
which  serve  to  characterize  genera,  species,  and  individuals. 
Furthermore,  since  the  factors  which  give  rise  to  constitutional 
changes  in  one  vital  substance  would  probably  operate  at  the 
same  time  to  cause  related  changes  in  certain  others,  the  alterations 
21 


322  BIOLOGY:  GENERAL   AND  MEDICAL 

in  one  may  logically  be  assumed  to  serve  as  a  common  index  of  all. 
In  accordance  with  the  foregoing  statement  it  can  readily  be 
understood  how  environment,  for  instance,  might  so  affect  the 
individual's  metabolic  processes  as  to  give  rise  to  modifications  of 
the  constitutions  of  certain  corresponding  proteins  and  other 
vital  molecules  which,  even  though  they  be  of  too  subtle  a  character 
for  the  chemist  to  detect  by  his  present  methods,  may  nevertheless 
be  sufficient  to  cause  not  only  physiological  and  morphological 
differentiations  in  the  individual,  but  also  become  manifested 
physiologically  and  morphologically  in  the  offspring." 

"Furthermore,  if  the  corresponding  proteins  and  other  complex 
organic  structural  units  of  the  different  forms  of  protoplasm  are 
not  identical  in  chemical  constitution  it  would  seem  to  follow,  as  a 
corollary,  that  the  homologous  organic  metabolites  should  have 
specific,  dependent  differences.  If  this  be  so,  it  is  obvious  that 
such  differences  should  constitute  a  pre-eminently  important 
means  of  determining  the  structural  and  physiological  peculiarities 
of  protoplasm." 

"To  what  extent  this  hypothesis  is  well-founded  may  be  judged 
from  this  partial  report  of  the  results  of  our  investigation:  It 
has  been  conclusively  shown  not  only  that  corresponding  hemo- 
globins are  not  identical,  but  also  that  their  peculiarities  are  of  a 
positive  generic  specificity,  and  even  much  more  sensitive  in  their 
differentiations  than  the  "  zooprecipitin  test."  Moreover,  it  has 
been  found  that  one  can  with  some  certainty  predict  by  these 
peculiarities,  without  previous  knowledge  of  the  species  from  which 
the  hemoglobins  were  derived,  whether  or  not  interbreeding  is 
probable  or  possible,  and  also  certain  characteristics  of  habit,  etc." 

"The  question  of  interbreeding  has,  for  instance,  seemed 
perfectly  clear  in  the  case  of  Canidce  and  Muridce,  and  no  difficulty 
was  experienced  in  forecasting  similarities  and  dissimilarities  of 
habit  in  Sciuridce,  Muridce,  Felidce,  etc.,  not  because  it  is  per  se 
the  determining  factor,  but  because,  according  to  this  hypothesis, 
it  serves  as  an  index  (gross  though  it  be,  with  our  present  very 
limited  knowledge)  of  those  physico-chemical  properties  which 
serve  directly  or  indirectly  to  differentiate  genera,  species,  and 
individuals.  In  other  words,  vital  peculiarities  may  be  resolved  to 
a  physico-chemical  basis." 

Though  it  is  naturally  much  more  difficult  to  deter- 
mine with  reference  to  the  cells,  there  are  reasons  for 
believing  that  the  cell  juices,  like  the  body  juices,  differ 
among  different  kinds  of  animals.  Evidences  of  this 
are  seen,  for  example,  in  the  destruction  of  the  alien  cor- 
puscles in  cases  of  transfusion  with  heterologous  blood. 
The  desire  to  make  good  the  blood  lost  by  hemorrhage, 
that  first  prompted  physicians  to  transfuse  the  blood  of  a 
sheep  or  other  lower  animal  into  the  exsanguinated  human 
vessels,  was  based  upon  the  erroneous  assumption  that  red 
bloods  were  all  alike.  We  now  know  that  the  disappoint- 


BLOOD    RELATIONSHIP  323 

ing  results  following  such  treatment  depend  in  part  upon 
the  inappropriate  character  of  the  heterologous  blood  of 
which  the  patient  can  make  little  use,  and  which  places 
him  under  the  disadvantage  of  being  compelled  to  dis- 
solve and  destroy  all  the  formed  elements  as  well  as 
to  rid  himself  of  the  offensive  proteins  in  the  serum. 
It  has  been  found  by  experience  that  physiological  salt 
solution  and  Ringer's  solution  are  more  satisfactory  in 
that  they  fill  the  vessels,  and  enable  the  heart  to  con- 
tinue its  work  until  blood  regeneration  begins,  without 
introducing  anything  offensive  into  the  body.  But  when 
the  corpuscles  are  essential,  and  the  transfusion  is  practised 
for  the  purpose  of  supplying  the  anemic  patient  with 
them,  human  blood,  and  not  heterologous  blood,  must  be 
employed.  Under  these  circumstances,  when  the  blood 
of  one  healthy  human  being  is  introduced  into  the  vessels 
of  another,  it  would  seem  as  though  invariable  benefit 
ought  to  accrue,  but  the  result  is  riot  always  so  satisfactory 
as  might  be  expected,  because  there  are  individual  as  well 
as  specific  and  racial  differences,  and  the  normal  blood  of 
one  human  being  may  prove  prejudicial  to  another. 

Thus,  if  the  introduced  corpuscles,  instead  of  being  per- 
mitted to  circulate  like  those  normal  to  the  patient,  are 
agglutinated  or  dissolved,  no  benefit  can  be  derived  from 
their  presence,  and  the  patient  may  be  injured  through 
the  occurrence  of  fever,  edema,  hemoglobinuria,  etc. 

That  certain  individuals  are  benefited  and  others  in- 
jured by  human  corpuscles  vicariously  supplied  to  them 
has  been  shown  through  the  researches  of  Landsteiner, 
Shattuck,  Hektoen,  Ottenburg,  Moss,  Gay,  Brem,  Lee, 
Karstner,  and  others  to  depend  upon  the  presence  or 
absence  in  the  blood  of  the  recipient  of  factors  called 
isohemagglutinins  and  isohemolysins. 

If  these  factors  are  absent,  the  donated  corpuscles  are 
uninjured  and  seem  to  be  utilized  as  are  those  fabricated 
by  the  patient  himself;  if  they  are  present,  the  donated 
corpuscles  are  attacked  and  destroyed. 

As  the  outcome  of  the  investigations  to  which  reference 
has  been  made,  human  beings  can  be  divided  into  four 


324 


BIOLOGY:  GENERAL  AND  MEDICAL 


groups  according  to  the  interaction  of  their  serums  and 
red  corpuscles.  The  differences  make  their  appearance 
when  the  individuals  are  about  one  year  old,  and  are 
permanent  and  hereditary  The  groups  may  be  shown 
in  tabular  form  thus: 


Serums 
Groups     I          II        III 


IV 


Corpuscles 

I 
II 
III 
IV 

0 

+ 

+ 

+ 

0 

0 

+ 

+ 

0 
0 

+ 

0 

0 

+ 

0 

0 

By  examining  the  table  it  becomes  evident  that  the 
serums  of  members  of  Group  I  are  without  injurious 
effect  upon  any  corpuscles  and  that  the  individuals  of 
this  group  can,  therefore,  receive  them  from  any  donor. 
Cases  falling  in  Group  I  may,  therefore,  be  described  as 
"universal  recipients,"  and  Karstner  has  shown  that  about 
3.1  per  cent,  of  human  beings  are  so  constituted. 

It  is  also  evident  that  the  corpuscles  of  cases  falling  in 
Group  IV  are  not  affected  by  the  serums  of  any  of  the 
other  groups  and  can,  therefore,  be  given  to  any  patient 
needing  them.  Cases  falling  in  Group  IV  are,  therefore, 
capable  of  acting  as  "universal  donors,"  and  Karstner  finds 
that  about  46.2  per  cent,  of  human  beings  fall  into  this  class. 

Difficulty  arises  when  the  individuals  to  be  transfused 
or  the  donors  of  the  corpuscles  fall  into  Group  II,  of  which 
there  are  about  42.4  per  cent,  among  human  beings,  or 
Group  III,  of  which  there  are  about  8.3  per  cent,  of  human 
beings  according  to  the  same  author. 

Theoretically,  members  of  Group  II  can  receive  cor- 
puscles, with  benefit,  from  Groups  II  and  IV  only;  mem- 
bers of  Group  III,  from  Groups  III  and  IV  only,  and 
members  of  Group  IV,  from  Group  IV  only. 


BLOOD   RELATIONSHIP  325 

But,  practically,  the  matter  is  simplified  by  the  fact 
that  little  if  any  attention  need  be  paid  to  the  effect  of 
the  donor's  serum  upon  the  recipient's  .corpuscles.  It  is 
of  the  utmost  importance  to  know  that  the  recipient's 
serum  will  neither  agglutinate  nor  dissolve  the  donated 
corpuscles,  for  only  under  these  circumstances  should  the 
transfusion  be  effected.  It  is  of  far  less  importance  to 
know  that  the  donor's  serum  will  agglutinate  or  dissolve 
the  recipient's  corpuscles,  for  should  that  be  the  case,  the 
transfusion  may  be  practised,  seeing  that  the  quantity  of 
donated  blood,  not  being  more  than  one-tenth  or  one- 
fifth  of  the  total  blood  bulk,  meets  with  a  dilution  that 
usually  makes  the  offensive  action  of  the  serum  upon  the 
corpuscles  impossible. 

In  practice  it  is  not  necessary  to  know  to  which  of  the 
various  groups  the  recipient  and  donor  belong;  all  that  is 
necessary  is  to  find  out  by  experiment  that  when  a  drop 
of  blood  drawn  from  the  finger  of  the  donor  is  mixed  with 
an  equal  volume  of  the  serum  of  the  recipient  no  aggluti- 
nation or  solution  of  the  corpuscles  takes  place. 

When  the  subject  of  grafting  is  considered,  it  will  be 
found  that  the  blood  relationship  of  the  scion  and  the  stock 
in  both  plants  and  animals  probably  has  much  to  do 
with  determining  the  success  or  failure  of  the  experiment. 
Tissue  taken  from  one  animal  and  grafted  upon  another 
survives  or  is  destroyed  in  large  measure  according  to 
the  blood  relationship  of  the  animals  concerned.  So 
sensitive  are  the  tissues  in  this  particular  that,  among 
animals  successful  grafting  can  rarely  be  performed 
when  specific  differences  obtain  among  them. 

It  also  appears  as  though  this  matter  of  blood  relation- 
ship with  its  affinities,  indifferences,  or  repugnances 
may  explain  the  difficulties  attending  successful  hybridi- 
zation. The  germinal  cells  of  specifically  different 
organisms  doubtless  possess  the  same  sensitivity  to 
heterologous  cells  that  are  manifested  by  the  somatic 
cells,  so  that,  instead  of  fertilizing  one  another,  they 
remain  indifferent  to  one  another,  repel  one  another,  or 
perhaps  even  destroy  one  another. 


320  BIOLOGY:  GENERAL  AND  MEDICAL 

The  interesting  researches  of  Harrison,  Burrows,  and 
Carrel  upon  the  growth  of  tissues  in  vitro,  prosecuted  with 
much  zeal  during  the  years  1907  and  the  present  time, 
have  shown  that  many  of  the  normal  tissues  of  embryos 
and  adults  of  both  warm-  and  cold-blooded  animals  can  be 
successfully  cultivated  in  vitro,  in  homologous  plasma.  In 
the  embryonal  tissues  when  the  tendency  to  grow  is  most 
marked,  cultures  may  be  obtained  in  homologous  plasma,  in 
some  cases  in  heterologous  plasma,  rarely  in  homologous 
and  heterologous  serum,  and  in  a  few  cases  in  Ringer's  solu- 
tion; but  adult  tissues,  with  limited  capacity  for  growth  in 
comparison,  can  only  be  cultivated  in  homologous  plasma. 

Finally,  blood  relationship  has  a  distinct  bearing 
upon  the  subject  of  symbiosis  for  organisms  whose  chem- 
ical affinities  are  opposed  to  one  another  may  be  unable 
to  become  symbionts;  and  in  cases  of  parasitism,  it  is 
conceivable  that  one  of  the  first  adaptations  to  be  acquired 
by  the  parasite  is  tolerance  to  the  physiologico-chemically 
antagonistic  conditions  to  be  found  in  the  body  of  the  host. 
Indeed,  as  will  be  shown  in  the  chapter  upon  Infection 
and  Immunity,  the  experimental  exaltation  of  micro- 
organismal  virulence  is  a  matter  of  overcoming  the  body 
defenses,  which  from  the  present  standpoint  may  be  looked 
upon  as  the  establishment  of  a  tolerance  toward  originally 
antagonistic  chemico-physiological  conditions. 

REFERENCES. 

GEORGE  H.  F.  NUTTALL:  "  Blood  Immunity  and  Blood  Relation- 
ship," Cambridge,  1904. 

EDWARD  TYSON  REICHERT  and  AMOS  PEASLEE  BROWN:  "The 
Differentiation  and  Specificity  of  Corresponding  Proteins 
and  Other  Vital  Substances  in  Relation  to  Biological 
Classification  and  Organic  Evolution:  The  Crystal- 
lography of  Hemoglobins."  The  Carnegie  Institution, 
Washington,  1909. 

MAGNUS  and  FRIEDENTHAL:  "Ueber  die  Specifitat  der  Verwand- 
schaftsreaction  der  Pflanzen."  Berichte  der  deutschen 
botanischen  Gesellschaft,  xxv,  1907,  242;  xxiv,  1906,  601. 

Ross  G.  HARRISON:  Proc.  Soc.  Exp.  Biol.  and  Med.  1907,  iv,  140; 
Jour.  Exp.  Zool.,  1910,  ix.,  787. 

MONTROSE  T.  BURROWS:  Jour.  Amer.  Med.  Assoc.,  1910.,  Iv,  2057: 
Jour.  Exp.  Zool.,  1911,  x,  63. 

ALEXIS  CARREL:  Jour.  Exp.  Med.,  1911-1912. 

W.  L.  Moss:  Studies  on  Isoagglutinins  and  Isohemolysins,  Bulletin 
of  the  Johns  Hopkins  Hospital,  1910,  xxi,  63. 


CHAPTER  XIV. 
PARASITISM. 

A  simple  calculation  will  show  that  the  unrestricted 
multiplication  of  any  living  organism  would  cause  it  to 
cover  the  entire  surface  of  the  earth  in  the  course  of  a 
relatively  short  time.  No  organism  has,  however,  met 
with  such  numerical  success.  The  earth  is  tenanted  by 
a  highly  diversified  flora  and  fauna  in  which,  though 
certain  forms  greatly  preponderate  over  others  in  num- 
bers, all  are  restricted  by  certain  conditions  that  may 
not  be  overcome. 

The  first  of  these  has  to  do  with  the  unequal  condition 
of  the  earth's  surface  where  variations  of  temperature, 
moisture,  chemical  composition  of  the  soil,  exposure  to 
violent  winds  and  deluges  of  rain  determine  that  great 
numbers  of  living  things  of  many  different  kinds  shall 
thrive  in  hospitable  localities,  diminishing  numbers  in- 
habit less  hospitable  localities,  and  none  at  all  be  found 
in  the  least  hospitable  localities. 

The  second  have  to  do  with  the  behavior  of  the  living 
things  toward  one  another,  which,  though  appearing 
harmonious  upon  cursory  observation,  is  found  upon 
critical  examination  to  be  an  unending  interference 
which  Darwin  has  aptly  described  as  the  "  struggle  for 
existence." 

In  this  perpetual  strife  it  is  easily  conceived  that  those 
forms  best  adapted  to  the  exigencies  of  the  situation  will 
survive  while  their  less  fortunate  fellows  must  fail; 
hence  the  "struggle  for  existence"  results  in  the  "sur- 
vival of  the  fittest." 

These  subjects  are  treated  at  considerable  length  else- 
where, but  for  an  intelligent  understanding  of  the  subject 
under  consideration  it  is  important  to  start  with  the  idea 

327 


328  BIOLOGY:  GENERAL  AND  MEDICAL 

that  the  "struggle  for  existence"  is  at  the  bottom  of  the 
parasitic  association. 

In  a  general  way  dissimilar  living  things  are  indifferent 
to  one  another  provided  they  do  not  prey  upon  one 
another.  Sometimes,  however,  they  assume  an  intimacy 
so  close  that  where  one  is  found  the  other  can  always 
be  expected.  Under  such  circumstances  it  may  justly 
be  surmised  that  the  close  association  is  founded  upon 
some  mutual  advantage  that  brings  the  two  together. 

To  this  communion  of  life  interests  the  term  Symbiosis 
is  applied,  and  each  of  the  organisms  is  described  as  a 
symbiont.  In  some  cases  the  symbionts  are  so  closely 
united  as  to  appear  to  form  a  single  organism,  as  in  the 
lichens  which  consist  of  an  alga  upon  which  a  fungus 
grows.  This  is  described  as  conjunctive  symbiosis. 
When  the  symbionts  are  less  closely  blended,  the  relation- 
ship is- described  as  disjunctive  symbiosis. 

Symbiosis  may  be  further  subdivided  into  commen- 
sdlism,  mutualism,  helotism,  and  parasitism. 

Commensalism. — This  is  a  form  of  symbiosis  in  which 
the  symbionts  derive  no  known  advantage  from  their 
intimacy  nor  do  they  do  one  another  any  harm.  Most 
interesting  examples  are  available.  One  of  the  first  to 
suggest  itself  is  the  little  crab  so  commonly  found  in  the 
shell  of  the  oyster.  It  does  the  oyster  no  harm  nor 
does  it  derive  any  benefit  except  that  of  the  defense 
afforded  by  the  strong  shell  of  its  host.  When  the  shell 
is  open,  it  is  free  to  enter  and  exit  at  will;  when  it  is 
closed,  it  becomes  a  captive.  Another  perhaps  more 
interesting  example  is  the  sea  anemone  so  commonly 
found  upon  the  shell  of  the  hermit  crab.  In  some  of 
these  cases  the  anemone  is  attached  to  the  claw  of  the 
crab  and  serves  to  hide  the  animal  by  closing  the  door 
of  the  shell  when  it  retreats.  If  the  crab  is  able  to 
appreciate  this  advantage  it  may  explain  why,  when 
the  animal  sheds  and  is  obliged  to  seek  a  new  home, 
it  seizes  the  anemone,  tears  it  from  its  old  attachment, 
and  carries  it  off  with  it,  as  it  is  frequently  said  to  do. 
In  cases,  however,  hi  which  the  anemone  is  not  thus 


PARASITISM  329 

situated,  but  is  on  the  shell  forming  the  crab's  house, 
it  is  less  easy  to  understand  the  behavior  of  the  animal 
which  is  said  at  times  to  transfer  its  anemone  to  the 
new  shell  when  its  home  is  changed. 

Commensalism   among   the   higher  plants   might   be 
exemplified  by  the  epiphytes,  plants,  like  orchids,  that 


FIG.  109. — Mutualism  of  diatom  and  bacteria,     (Verworn.) 

cling  to  other  plants,  live  upon  them,  yet  not  at  their 
expense  or  to  their  injury.  In  the  tropics  scarcely  a 
tree  can  be  found  upon  which  many  varieties  are  not 
present. 

Mutualism. — This  is  a  form  of  symbiosis  in  which  one 
or  both  of  the  symbionts  derives  advantage  from  the 
association  without  injury  to  the  other.  The  advantage 
is  usually  physiological  in  character.  Thus,  certain 
radiolaria  not  infrequently  harbor  a  small  green  alga, 
the  Zooxanthella.  The  alga  gives  off  oxygen  that  is 
advantageous  to  the  little  animal,  which  in  turn  gives 


330  BIOLOGY:  GENERAL  AND  MEDICAL 

off  carbon  dioxide  that  is  useful  to  the  plant.     Here 
both  symbionts  are  benefited  by  the  association. 

The  symbiosis  of  the  alga  and  fungus  constituting  a 
lichen  is  a  form  of  mutualism  that  may  be  of  physiological 


-- 1 


t 


FIG.  110. — Tubercles,  on  the  roots  of  red  clover.  I,  Section  of  ascending 
branches;  6,  enlarged  base  of  stem;  t,  root  tubercles  containing  bacteria.  (From 
Bergen  and  Davis"  "Principles  of  Botany."  Ginn  &  Co.,  publishers.) 

benefit  to  both  symbionts,  the  fungus  furnishing  nitro- 
gen and  the  alga  carbohydrates. 

Again,  the  bacteria  that  form  tubercles  upon  the  root- 
lets of  the  leguminous  plants  are  of  great  benefit  to  them 
by  fixing  nitrogen.  At  the  same  time  they  probably 
derive  their  nutrition  from  the  juices  of  the  plant  with- 
out damaging  it. 

The  bacteria  that  live  upon  the  skin,  in  the  mouth,  and 
in  the  intestines  of  animals  are  for  the  most  part  harm- 
less mutuals  that  derive  their  nourishment  from  the 
host  without  causing  any  harm. 

Certain  bees  live  under  conditions  of  mutualism  as 


PARASITISM  331 

Psithyrus  in  the  nests  of  Bombus.  It  is  supposed  that 
the  only  benefit  Psithyrus  receives  is  the  shelter  of  the 
nest,  but  it  may  be  that  its  young  are  nourished  by  the 
bees'  honey.  In  return  for  this,  Psithyrus  aids  in 
defending  the  nest. 

Helotism. — In  this  form  of  symbiosis  the  one  organism 
is  supposed  to  enslave  the  other  and  enforce  it  to  labor 
in  its  behalf.  The  term  seems  to  be  susceptible  of  many 
applications  in  the  hands  of  different  writers.  Thus, 
by  some  it  is  said  that  among  the  lichens  the  algae  are 
enslaved  by  the  symbiotic  fungi.  A  better  example 
is  to  be  found  in  the  behavior  of  certain  ants  that  under- 
take systematic  campaigns  against  other  ants  bringing 
home  the  conquered  to  be  their  slaves.  The  enslaved 
ants  soon  seem  to  be  quite  at  home  and  maintain  a 
subsequent  harmonious  symbiotic  existence  in  the  nests 
of  their  masters  where  they  are  easily  recognized  by 
their  different  specific  characters.  So  far  as  known,  it  is 
only  the  worker  ants,  never  the  males  or  females,  that 
are  thus  enslaved,  and  having  no  sexual  instinct,  but 
only  the  instinct  to  labor,  after  the  heat  of  the  action  is 
over,  they  seem  to  be  as  contented  to  work  in  one  place 
as  in  another.  The  precaution  seems  to  be  taken,  how- 
ever, to  have  them  participate  in  the  household  duties 
rather  than  to  engage  in  foraging  expeditions  or  to  join 
the  ranks  in  warfare. 

Parasitism. — This  is  a  form  of  symbiosis  in  which  one 
symbiont  receives  advantage  to  the  detriment  of  the 
other.  The  symbiont  receiving  the  advantage  is  known 
as  the  parasite,  the  other  as  the  host. 

The  parasitic  relationship  is  based  upon  the  ease  with 
which  the  products  of  another's  labor  can  be  seized  upon 
to  the  saving  of  one's  own  expenditure.  In  most  cases, 
therefore,  the  parasite  is  the  lazy  creature  that  lives  at 
another's  cost.  In  general  it  is  a  miserable  form  of  ex- 
istence characterized  by  many  vicissitudes  and  marked 
decadence  of  the  parasitic  forms.  The  actual  decadence 
of  the  parasites,  however,  depends  upon  the  nature  of  the 
symbiotic  relationship.  When  this  is  disjunctive  the 


332  BIOLOGY:  GENERAL  AND  MEDICAL 

parasites  maintain  a  certain  degree  of  independence, 
but  when  it  is  conjunctive  they  must  live  and  die  with 
the  host. 

The  parasitic  organisms  include  representatives  of 
both  cryptogams  and  phanerogams  among  plants,  and 
of  nearly  all  of  the  phyla  of  animals  from  the  protozoa 
to  the  vertebrata. 

According  to  their  habits,  they  may  be  described  as 
occasional  or  optional  parasites  and  obligatory  parasites. 
With  the  adoption  of  the  parasitic  mode  of  life  struc- 
tural modifications  usually  make  their  appearance  so  as 
to  adapt  the  organism  to  its  environment,  after  which  its 
new  mode  of  life  becomes  obligatory. 

Thus,  the  mosquito  may  be  cited  as  an  example  of  the 
occasional  parasite.  Under  conditions  prevailing  in 
many  localities  where  mosquitoes  abound,  these  insects 
live  by  sucking  the  nectar  of  flowers  and  the  juices  of 
plants.  They  have,  however,  a  marked  preference  for 
the  blood  of  animals  and  a  few  cannot  ovulate  except 
after  a- meal  of  warm  blood. 

The  next  step  in  the  direction  of  obligatory  parasitism 
is  seen  among  organisms  that  visit  the  hosts  occasionally 
though  absolutely  dependent  upon  them  for  the  means 
of  subsistence.  Among  such  we  find  the  biting  flies, 
the  fleas,  and  the  bed-bugs.  The  latter  form  excellent 
examples,  for  they  do  not  live  upon  the  host,  but  inhabit 
crevices  in  the  bed  or  cracks  in  the  walls  and  sally  forth  at 
night  to  feed.  Their  mouth  parts  are  so  constructed  that 
it  is  impossible  for  them  to  feed  in  any  other  way,  and  if 
the  host  should  vacate  his  habitation  the  bugs  must 
inevitably  die,  unless  in  the  absence  of  human  tenants 
they  can  make  shift  with  some  other  warm-blooded 
animal  that  may  become  available.  Fleas  leap  upon 
their  appropriate  hosts,  sometimes  to  satisfy  their  appe- 
tites, sometimes  to  remain.  Should  death  overtake  the 
particular  host,  they  desert  him  for  another,  but  should 
none  be  available,  they  too  must  die,  having  mouth 
parts  solely  adapted  for  sucking  blood. 

The  final  step  is  reached  with  the  lice.     One  species 


PARASITISM  333 

inhabits  the  clothing  and  visits  the  skin  to  feed;  other 
species  attach  themselves  to  the  hairs  and  are  permanent 
as  well  as  obligatory  parasites. 

All  the  parasites  of  this  class;  whether  they  visit  the 
surface  of  the  body  occasionally,  attach  themselves  to  it 
permanently,  or  even  burrow  into  it,  like  the  Sarcoptes 
scabei,  or  itch  mite,  are  ectoparasites.  A  still  more  close 
relationship — conjunctive  symbiosis — is  seen  in  those 
cases  in  which  the  parasite  actually  enters  the  body  of 
the  host  and  inhabits  its  blood,  tissues,  or  alimentary 
canal.  Such  are  known  as  endoparasites. 

Many  endoparasites  have  complicated  Me  histories, 
seemingly  necessitated  by  the  difficulty  of  ensuring  suc- 
cessive generations,  and  commonly  live  different  stages 
of  their  existence  in  different  hosts.  These  may  be  ani- 
mals of  the  same  kind,  but  are  more  commonly  of  widely 
differing  kinds.  The  host  harboring  the  embryonal,  larval, 
or  asexual  stage  of  any  parasite  is  called  the  intermediate 
host;  that  harboring  the  adult  or  sexual  stage,  the  defini- 
tive host. 

Parasitic  symbiosis  not  only  takes  place  by  design, 
but  also  by  accident,  and  indeed  since  it  may  be  con- 
jectured that  accident  first  brought  about  and  fostered 
the  relationship,  it  is  difficult  in  some  cases  to  say 
whether  we  are  dealing  with  true  parasitism  or  not. 
Among  the  infectious  diseases  of  man  and  animals,  and 
indeed  of  plants,  we  are  not  infrequently  puzzled  to 
know  whether  certain  bacteria  are  parasites  in  the  true 
sense  or  not.  Thus,  the  bacillus  of  tetanus  or  lock-jaw 
is  a  rather  common  tenant  of  the  alimentary  apparatus 
of  herbivorous  animals  with  whose  dejecta  it  finds  its 
way  to  the  soil  in  which  it  is  ^sometimes  found  in  large 
numbers.  *  When  this  bacillus  is  accidentally  admitted 
to  the  tissues  of  certain  animals,  it  proceeds  to  live  and 
multiply,  and  eventually,  in  many  cases,  to  destroy  the 
animal  through  the  virulence  of  its  toxic  metabolic 
products.  Here  the  accidental  circumstance  of  a  wound 
leads  to  an  unexpected  symbiosis  that  is  fatal  to  the  host 
and  later  to  the  parasite  as  well. 


BIOLOGY:  GENERAL  AND  MEDICAL 


When  this  circumstance  is  carefully  analyzed  we  find 
that  one  of  the  fundamental  conditions  of  true  parasit- 
ism is  wanting,  for  no  means  is  provided  for  the  future. 
The  host  dies  before  the  bacilli  have  become  numerous, 
there  is  little  multiplication  of  the  bacilli  in  the  body 
after  death,  and  no  means  is  provided  by  which  they 
shall  leave  the  host  to  find  their  way  to  another. 

The  case  is  quite  different  with 
other  bacteria.  Thus,  the  bacillus 
of  tuberculosis  is  unknown  in  na- 
ture except  in  the  disease  it  causes. 
The  microorganisms  are  eliminated 
from  the  body  of  the  diseased  in 
enormous  numbers  through  morbid 
discharges  and  find  their  way  to 
new  hosts  through  the  association 
of  the  well  with  the  ill.  Here  there 
is  little  doubt  of  the  parasitic  nature 
of  the  symbiosis. 

In  certain  cases  accident  may 
transform  the  symbiotic  relation- 
ship from  harmless  to  harmful. 
Thus  the  colon  bacillus  is  to  be 
found  in  nearly  all  vertebrates, 
whose  alimentary  tract  it  inhabits 
to  feed  upon  the  nutritious  con- 
tents. When  local  disease  of  the 
intestine  arises  from  any  cause,  in- 
vasion of  the  tissues  by  the  bacilli 
may  supervene,  and  the  usually 
inoffensive  organism  may  occasion 
disturbances  resulting  in  the  death 
of  the  host. 

Plant  parasites  are  innumerable 
and  attack  animals  as  well  as  other 

plants.  Bacteria  are  not  infrequently  parasitic  upon 
higher  plants,  an  excellent  example  being  found  in  the 
wilt  disease  of  cucumbers  and  melons.  The  entire  class 
of  fungi  is  composed  of  organisms  that  must  derive  their 


FIG.  111.— The  ergot  of 
rye,  Claviceps  purpurea. 
Ear  of  rye  showing  two 
sclerotia  of  the  fungus.  2/3 
natural  size.  (Partly  after 
Tulasne.)  (Kerner  and 
Oliver.) 


PARASITISM 


335 


nourishment  from  other  organisms,  either  living  or  dead, 
and  so  comprehends  an  enormous  number  of  parasites. 
Of  these,  familiar  examples  will  be  the  "smut"  upon 


ABC 

FIG.  112. — Dodder,  a  parasitic  seed  plant.  A,  Magnified  section  of  stem 
penetrated  by  roots  of  dodder;  B,  dodder  upon  a  golden- rod  stem;  C,  seedling 
dodder  plants  growing  in  earth;  h,  stem  of  host;  /,  scale-like  leaves;  r,  sucking 
roots,  or  hanstoria;  s,  seedlings.  (A  and  C  after  Strasburqer.  From  Bergen 
and  Davis'  "Principles  of  Botany."  Ginn  &  Co.,  publishers.) 


corn  and  rye,  the  potato  "rot,"   the  grape-vine  "mil- 
dew," the  various  "rusts,"  and  some  of  the  leaf  curls. 


336 


BIOLOGY:  GENERAL  AND  MEDICAL 


Upon  the  roots  of  the  Cycas,  or  sago  palm,  certain 
species  of  Anabaena  are  always  symbiotic  and  perhaps 
parasitic. 

Higher  plants  may  also  adopt  the  parasitic  life.  The 
"dodders,"  so  often  found  upon  meadow  plants,  are 
typical  parasites.  From  a  seed  that  falls  upon  the  ground 
a  delicate  filament  makes  its  appearance  climbing  like 
a  vine  about  the  stems  of  some  other  plant.  Soon  deli- 
cate rootlets  grow  into  the  tissues  of  the  host,  and  the 


FIG.  113. — The  European  mistletoe  (Viscum  album).     (Kt 


Oliver.) 


primitive  root  of  the  parasite  dies.  As  it  now  derives 
its  entire  sustenance  through  the  rootlets  that  have 
penetrated  the  plant  to  which  it  clings,  it  can  dispense 
with  organs  of  its  own  and  has  neither  terrestrial  roots 
nor  leaves.  It  grows  rapidly,  forming  a  pale  colored . 
tangled  filamentous  mass  that  bears  abundant  small 
flowers  in  clusters,  and  produces  small  seeds. 


PARASITISM 


337 


Other  striking  examples  are  the  mistletoes.  These 
plants,  of  which  hundreds  of  species  are  known,  produce 
berries  full  of  a  very  viscid  juice  which  serves  to  glue  the 
seeds  to  the  branches  of  the  trees  upon  which  they 
germinate,  the  little  rootlets  striking  upward  and  pene- 
trating into  the  tissues  of  the  host  from  which  the  para- 
site derives  its  nourishment.  The  stem  continually 
forks  dichotomously,  each  branch  terminating  in  a 
pair  of  fleshy  leaves.  The  flowers  are  small  and  appear 
at  the  ends  of  the  branches  and  in  the  small  divisions. 
As  the  plant  is  evergreen  it  forms  a  striking  object  in 
winter  when  it  appears  as  a  thick  tangle  of  tiny  green 


FIG.  114. — Bastard  toad-flax  (Thesium  alpinum).  1.  Root  with  suckere; 
almost  natural  size;  2,  piece  of  a  root,  with  sucker  in  section.  X  about  35' 
(Kerner  and  Oliver.) 

leaves   and   stems  upon   the   otherwise  bare   branches 
of  many  common  trees. 

A  peculiar  form  of  vegetable  parasite  is  the  "bastard 
toadflax,"  a  rather  pretty  flowering  plant.  What  can  be 
seen  above  ground  is  an  independent  plant  with  stem, 
leaves,  and  flowers  of  its  own,  but  below  ground  its  roots 
attach  themselves  to  the  neighboring  roots  of  other  plants 
which  it  robs  and  thus  dwarfs. 


338  BIOLOGY:  GENERAL  AND  MEDICAL 

Among  animal  organisms  nearly  every  phylum  has 
its  parasitic  representatives.  They  are  most  numerous 
among  the  most  simple  organisms  and  occur  with  dimin- 
ishing frequency  as  the  scale  of  life  is  ascended  until, 
when  the  vertebrates  are  reached,  there  is  but  a  single 
representative. 

The  following  systematic  arrangement  of  the  parasitic 
forms  is  founded  upon  the  excellent  paper  upon  the  sub- 
ject in  the  New  International  Encyclopedia. 
Phylum — Protozoa . 

Class — Rhizopoda. — These  include  the  amoeba, 
of  which  many  species  are  parasitic  in  the 
alimentary  tracts  of  many  other  animals.  The 
most  important  is  the  Entamceba  histolytica 
Schaudinn,  that  causes  tropical  dysentery 
in  man. 

Class — Infusoria. — These  are  ciliated  organisms 
like  Balantidium  coli  of  man.  Trichodinse  is  a 
parasite  of  the  gills  and  gill  cavity  of  the  frog; 
Opalina,  of  the  bladder  and  gut  of  the  frog; 
Holophyra,  an  epiparasite  of  certain  fish;  Ancis- 
trum  infests  the  mantle  cavity  of  certain  mol- 
lusks;  Anophlophyra  occurs  in  the  intestines  of 
certain  marine  invertebrates. 
Class — Mastigophora. — These  are  flagellate  or- 
ganisms like  Trichomonas  intestinalis,  Tricho- 
monas  vaginalis,  Cercomonas  intestinalis,  and 
Megastoma  entericum.  They  also  include  a 
number  of  alimentary  and  blood  parasites,  such 
as  Herpetomonas,  and  true  blood  parasites, 
such  as  the  Trypanosomes,  many  of  which  are 
dangerous  or  fatal  parasites  of  man  and  the 
lower  animals. 

Piroplasma  or  Babesia  and  the  Leishmania  may 
also  belong  to  this  group.  Many  are  harmful 
and  fatal  parasites  of  man  and  the  lower  ani- 
mals. 


PARASITISM  339 

Class — Sporozoa. — These  form  a  large  group  of 
mtracellular  parasites,  some,  as  Coccidia,  being 
parasites  of  epithelial  cells,  others,  like  Plasmo- 
dium,  blood  parasites,  and  still  others,  like  the 
Sarcocystis,  muscle  cell  parasites. 

Phylum — Porifera. — There    seem    to    be    no    parasitic 
forms  in  this  phylum. 

Phylum — Ccelenterata. — This    furnishes  very  few  para- 
sitic representatives,  none  of  which  affects  man. 

Class — Hydrozoa. — The  Polypodium  hydriforme 

at  one  stage  of  its  life  cycle  is  parasitic  upon 

the  immature  eggs  of  the  sturgeon..    Cunina  is 

parasitic  in  medusae. 

Class — Scyphozoa. — Probably  has   no  parasitic 

representatives. 

Class — Actinozoa — Edwardsia    is    parasitic  in 

Ctenophora;     Pemmatodiscus  on  Rhizostoma. 

Class — Ctenophora. — Gastroides  is  parasitic  in 

Salpa. 

Phylum — Echinodermata. — Probably  has  no  parasitic 

representatives. 

Phylum — Platyhelminthes. — Among   these   worms  are 

many   parasitic   individuals   infesting    man    and    lower 

animals. 

Class — Trematoda. — These  worms  are  all  para- 
sitic. The  best  known  are  the  liver  flukes, 
Fasciola  hepatlcum  and  Dicroccelium  lanceo- 
latum;  the  blood  fluke  Bilharzia  hematobium 
and  the  lung  fluke  Paragonimus  westermanii. 
All  of  these  are  parasites  of  man.  In  addi- 
tion there  are  many  others  that  infest  the 
lower  animals.  Some  forms  require  but  one 
host,  some  an  alternation  of  hosts. 
Class — Cestoda. — These  are  the  tape-worms, 
all  of  which  are  parasitic.  Of  those  infesting 
man  the  best  known  are  Taenia  saginata, 
Taenia  solium,  Dibothriocephalus  latus,  Hy- 
menolepis  nana,  Dipylidium  caninum,  and 


310  BIOLOGY:  GENERAL  AND  MEDICAL 

TaBnia  echinococcus.  All  of  these  have  com- 
plex life  histories  requiring  an  alternation  of 
hosts. 

Class — Turbellaria. — Of    these   Rhabdoccela  is 
parasitic  in  the  kidney  of  certain  gasteropods; 
Fecampia,  in  the  gut  of  certain  crabs 
Phylum — Nemathelminthes. 

Class — Nemertinea. — Of  these  worms  few  are 
parasitic.  Malacobdella,  however,  is  parasitic 
in  Lamellibranchs. 

Class — Nematoda. — These  are  the  round  worms, 
of  which  some  species  live  independent  lives, 
though  perhaps  the  greater  number  are  parasitic. 
A  few  infest  plants,  most  of  them  animals. 
Their  distribution  is  so  broad  that  few  of  the 
higher  animals  escape  them,  and  they  extend 
from  the  arthropoda  through  the  whole  series 
to  man  himself.  Of  the  human  parasites  of 
this  class  the  Ascaris  lumbricoides  or  round 
worm,  the  Oxyuris  vermicularis  orpin-worm,  the 
Anchylostoma  duodenale  or  the  palisade  worm, 
the  Necator  americana  or  hook-worm,  and  the 
Trichocephalus  dispar  or  whip-worm  are  intes- 
tinal parasites.  Trichinella  spiralis  is  at  one 
stage  an  intestinal  parasite,  but  later  a  muscle 
parasite.  Filaria  bancrofti  is  a  blood  parasite 
and  Filaria  medinensis,  a  tissue  parasite.  All 
are  of  rather  frequent  occurrence.  It  is  not  in 
all  cases  possible  to  separate  commensalism 
and  mutualism  from  parasitism  in  discussing 
these  worms. 

Class — Acanthocephala. — This  contains  four 
genera  and  a  number  of  species,  all  of  which  are 
parasitic.  They  infest  arthropods  for  the  most 
part  during  the  immature  stage,  the  adults 
appearing  in  vertebrates,  especially  fishes 
where  they  frequent  the  intestine.  They  are 
rarely  found  in  man. 


PARASITISM 


341 


Phylum — Trochelminthes. — These  comprise  the  rotifers 
or  wheel  animalcules,  and  are  rarely  parasitic. 

Class — Rotifer  a. — Of  these  a  few  species  are 
parasitic  in  Crustacea. 

Phylum — Annulata. — Of  the  segmented  worms  few  are 
parasitic. 

Class — Archi-annelida. — These  embrace  a  num- 
ber of  parasitic  forms,  none  of  which  infests 
man. 

Class — Discophora. — These  embrace  the  leeches 
which  may  be  included  among  the  occasional  or 
optional  parasites.  They  live  by  attaching 
themselves  to  fishes  and  sometimes  to  warm 
blooded  animals,  sucking  blood  until  distended, 
then  letting  go  again.  They  are  parasitic  in 
the  same  manner  as  bed-bugs  and  fleas. 
Phylum  —  Mollusca. — Of  the  mollusks  very 
few  are  parasitic. 

Class — Gasteropoda. — Of  these  Eulima,  Stylifer, 
and  Thyca  are  parasitic  in  Holothurians, 
star-fishes,  and  Echinoids,  embedding  them- 
selves in  the  skin  where  their  presence  occasions 
growths  resembling  tumors. 

Phylum — Arthropoda, — This  is  a  phylum  rich  in  para- 
sitic forms  of  great  interest. 
Class — Crustacea  : 

Entomostraca. — These  include  the  water 
fleas  or  Copepods  of  which  many  are 
parasitic.  Argulus  is  an  epiparasite  of 
fishes,  boring  between  their  scales;  Caligus 
is  parasitic  upon  the  gills  of  fishes;  Lera- 
conema  an  endoparasite  of  the  muscle  of 
fishes.  These  parasites  are  very  harm- 
ful to  their  respective  hosts. 
Another  group,  of  which  the  barnacles,  the 
Cirripedia,  are  well-known  members,  fur- 
nish a  few  parasitic  forms  that  attach 
themselves  to  the  abdomens  of  crabs. 


342  BIOLOGY:  GENERAL  AND  MEDICAL 

Arthrostraca. — Of  these  the  Amphipoda 
present  a  few  forms — Cyamus — that  are 
parasites  of  whales,  and  of  the  Isopoda, 
many  forms  that  are  parasitic  upon  the 
gills  or  scales  of  fishes. 

Class — Hexapoda — Insects. — Of  parasitic  in- 
sects there  seems  to  be  no  end,  nearly  every 
order  appearing  to  be  represented  in  some  form 
of  injurious  symbiosis  with  other  insects  or 
upon  other  animals  or  plants. 

Order — Mallophoga. — All  of  the  insects  of  this 
order  are  parasitic  upon  birds  and  mammals, 
the  symbiosis  being  conjunctive.  They  in 
general  resemble  lice,  are  without  wings,  have 
biting  mouth-parts,  not  fitted  for  sucking 
blood,  and  live  by  eating  the  feathers  and  hairs. 
Five  species  belonging  to  three  genera  are 
known  to  infest  the  barnyard  fowl.  Other 
species  infest  other  birds.  Certain  species 
sometimes,  but  rarely,  infest  the  dog  and  cat. 
Order — Hemiptera.—This  order  includes  the 
"bugs"  and  true  lice,  the  plant  lice  and  the 
scale  insects.  The  mouth-parts  of  the  entire 
group  are  fitted  for  piercing  and  sucking. 
Most  of  them  live  by  sucking  vegetable  juices; 
some  are  predatory  and  seize  upon  other  insects, 
sucking  their  blood;  some,  like  the  lice  and  bed- 
bugs, suck  the  blood  of  warm-blooded  animals. 
The  scale  insects,  red-bugs,  and  plant  lice  do 
great  damage  to  crops. 

Order — Lepidoptera. — This  order  which  com- 
prises the  butterflies  and  moths  includes  a  great 
number  of  representatives  that  are  parasitic 
upon  plants  in  the  larval  state.  One  is  a 
parasite  of  bees'  nests,  devouring  the  wax  and 
so  ruining  the  combs.  The  family  Epiphy- 
ropidse  have  larvae  that  live  upon  the  backs  of 


PARASITISM 


343 


leaf-hoppers  (Homoptera),  eating  the  sugary 
and  waxy  secretions  discharged  by  the  hosts. 
Order — Diptera. — This  large  order,  the  flies, 
contains  many  representatives  of  strikingly 
different  appearance,  whose  mouth-parts  are 
fitted  for  sucking  the  juice  of  plants  or  the  blood 
of  animals,  either  in  the  pupa  or  imago  stages. 
It  also  contains  quite  a  number  whose  habits 


FIG.  115. — Botfly  in  stomach  of  horse,  also  adult  fly.  (After  Michener, 
Report  on  Diseases  of  the  Horse,  U.  S.  Department  of  Agriculture,  Bureau  of 
Animal  Industry.) 

of  ovulation  are  parasitic.  A  few  forms  are 
obligatory  external  symbionts,  as  Melophagus 
ovinus,  the  sheep  tick. 

The  flesh  flies  not  infrequently  lay  their  eggs 
upon  wounds  and  in  discharging  openings  of 
the  bodies  of  animals  where  the  maggots  work 
their  way  into  the  tissues.  Though  most 
cases  of  myiasis  seem  to  be  by  accident  rather 


344  BIOLOGY:  GENERAL  AND  MEDICAL 

than  by  design,  the  "screw-worm"  appears  to 
prefer  animal  tissues  to  other  available  food 
for  its  young. 

The  botfly  fastens  its  eggs  to  the  hairs  of  horses, 
from  which  position  they  are  licked  off  and 
swallowed,  to  hatch  in  the  animal's  stomach. 
The  larvae  attach  themselves  to  the  mucous 
membrane,  where  they  remain  until  ready  to 
pupate,  when  they  let  go,  pass  through  the 
remainder  of  the  alimentary  canal,  and  dig  into 
the  ground  in  which  they  pupate,  and  from  which 
they  emerge  as  flies  after  a  time. 
The  sheep  bot  lays  her  eggs  at  the  nostrils  of 
the  sheep,  from  which  the  hatched  larvse  ascend 
to  the  frontal  sinuses  where  their  presence  causes 
vertigo  and  sometimes  the  death  of  the  sheep. 
When  the  larvae  are  mature  they  escape  from 
the  nose  and  pupate  on  the  ground. 
Botflies  of  the  genus  Hypoderma  sometimes 
form  tumors  through  irritation  of  the  skin  in 
which  the  larvse  develop.  The  "ox-warble," 
like  the  horse  bot,  lays  its  eggs  on  the  skin, 
from  which  they  are  licked  and  swallowed. 
The  larvae  make  their  way  through  the  oeso- 
phagus, to  the  subcutaneous  tissue,  where  they 
cause  tumor-like  swellings  from  which  they  bore 
out  at  maturity,  leaving  permanent  holes  in  the 
animal's  hide. 

Many  biting  flies,  Stomoxys,  Glossina,  etc.,  not 
only  annoy  warm-blooded  animals  by  biting 
them,  but  act  as  hosts  of  parasites  which  they 
take  from  the  blood  of  one  animal,  and  pass  to 
healthy  animals  subsequently  bitten.  The 
discovery  that  the  trypanosomes  of  "  tsetse 
fly  disease"  of  animals  and  " sleeping  sickness" 
of  man  are  spread  by  the  Glossina,  and  that 
those  of  "surra"  and  mat  de  caderas  are  simi- 
larly spread  by  Stomoxys  is  of  marked  economic 


PLATE   3 


Holcaxpis  (jlobulus.     A,  Galls  on  oak,  natural  size;  B,  the  gall-maker, 
twice  natural  length  (Folsom}. 


A  tomato  worm,  Phlegethontius  sexta,  bearing  cocoons  of  the  parasitic 
Apanteles  congregcdus.     Natural  size  (Folsom). 


PARASITISM  345 

importance,  since  means  of  inhibiting  the  multi- 
plication or  development  of  the  flies  may  go  far 
in  lessening  the  incidence  of  these  destructive 
maladies. 

In  the  same  manner  mosquitoes  have  been 
shown  not  only  to  be  optional  parasites  them- 
selves, but  also  to  act  as  definitive  hosts  of 
other  parasites  that  they  transmit  from  indi- 
vidual to  individual.  Such  parasitic  diseases 
of  man  as  paludism  or  malaria,  yellow-fever,  and 
filariasis  are  thus  transmitted  and  suspicions 
are  abroad  that  other  diseases  may  be  similarly 
spread. 

It  was  only  intelligent  understanding  of  the 
relation  of  the  mosquito  to  yellow  fever  that 
permitted  the  eradication  of  the  disease  from 
Havana  and  Panama. 

Dipterous  insects  are  also  important  plant 
parasites  sometimes  penetrating  the  tissues  in 
the  larval  state  to  complete  their  metamorpho- 
sis, sometimes  stinging  the  plant  during  ovu- 
lation  and  causing  the  formation  of  galls  or 
tumors  upon  the  tissues  of  which  the  larvae  live. 
Order — Siphonaptera. — This  order  includes  the 
fleas,  of  which  there  are  many  species  peculiar 
to  different  animals,  though  occasionally  in 
case  of  necessity  feeding  promiscuously  upon 
warm-blooded  creatures.  The  symbiotic  rela- 
tionship varies  in  closeness  in  different  cases. 
Upon  hairy  animals  the  fleas  remain  in  more 
intimate  association  than  upon  man.  The 
fleas  have  a  disposition  to  leave  the  host  occa- 
sionally and  hop  about  the  ground,  perhaps  to 
find  new  hosts.  The  parasitic  life  only  apper- 
tains to  the  adult  stage,  the  larval  and  pupal 
stages  occurring  upon  the  ground  where  vege- 
table food  is  consumed.  One  flea,  the  Sarcop- 
sylla  penetrans,  is  peculiar  in  that  the  female 


346  BIOLOGY:  GENERAL  AND  MEDICAL 

*  buries  herself  beneath  the  skin  of  the  host, 
the  abdomen  then  swelling  to  great  size  and 
causing  itching  papules  or  "  chiggerbuttons " 
to  form  upon  the  skin. 

Order — Coleoptera. — Of  these  insects,  the  bee- 
tles, a  vast  number  are  parasitic,  in  some 
or  all  of  their  stages,  upon  plants,  every  imagin- 
able structure  being  attacked  by  some  kind  of 
beetle. 

Beetles  are,  however,  very  rarely  parasites  of 
higher  animals  and  rarely  parasites  of  other 
insects.  Platypsyllus  castris  is  an  external  para- 
site of  the  beaver. 

Order — Strepsiptera. — An  interesting  example  of 
the  parasitic  organisms  of  this  order  is  the 
Stylops,  which  lives  between  the  abdominal 
plates  of  certain  hymenopterous  insects,  the 
abdomen  out  of  sight,  the  head  peeping  out. 
Order — Hymenoptera. — This  great  order  of  in- 
sects furnishes  the  greatest  numbers  and  most 
interesting  examples  of  parasitism.  In  the 
family  Ichneumonidae  more  than  ten  thousand 
parasitic  species  are  already  known,  with  acces- 
sions constantly  being  made. 
Some  of  the  hymenoptera  are  plant  parasites 
and,  like  the  dipterous  insects  of  similar  habits, 
sting  the  plants  at  the  time  of  oviposition  in 
such  manner  as  to  form  galls  and  other  excres- 
cences, upon  the  tissue  of  which  the  larvae  feed. 
The  usual  habit  of  the  hymenopterous  parasites 
is  to  ovulate  in  the  larvaB  and  pupae  of  other 
insects,  the  eggs  hatching  in  their  bodies  and 
the  larva?  feeding  upon  the  blood  and  fat  until 
ready  to  pupate  themselves,  when  they  usually 
bore  their  way  out  and  spin  silken  cocoons,  in 
which  they  mature.  A  familiar  example  of 
such  a  hymenopterous  parasite  is  found  in 
Apanteles  congregatus,  which  tiny  insect  lays 


PARASITISM  347 

its  eggs  in  the  tomato  worm  and  other  Sphin- 
gidae.  The  eggs  hatch,  the  larvae  live  upon  the 
fat  and  tissues  until  mature,  when  they  bore 
through  the  skin  and  spin  tiny  cocoons  which 
the  caterpillar  carries  fastened  to  its  back,  giving 
its  body  the  appearance  of  being  covered  with 
rice  grains. 

Many  species  are  provided  with  remarkably  long 
ovipositors  with  which  to  reach  and  lay  their  eggs 
in  larvae,  which  may  lie  hidden  away  under  bark, 
in  burrows  in  wood,  or  in  tunnels  in  the  earth,  or 
with  which  they  bore  through  cocoons  to  reach 
the  insect  hosts  at  the  beginning  of  the  stage  of 
pupation. 

Some  of  these  insects  (Megarhyssa  lunator  and 
Pimpla)  are  large  and  robust,  measuring  18  to  20 
mm.,  and  are  provided  with  ovipositors,  10  cm.  or 
more  in  length;  others,  the  Proctotrypidae,  are  so 
tiny  as  to  be  enumerated  among  the  smallest 
known  insects.  Such  minute  insects  oviposit  in 
the  eggs  of  other  insects  and  of  spiders. 
The  hymenopterous  parasites  are  of  immense 
benefit  to  man  by  holding  in  check  his  chief 
insect  enemies,  especially  coleopterous  and  lepi- 
dopterous  insects.  For  the  destruction  of  the 
cotton  worm  and  the  Hessian  fly  we  are  entirely 
under  obligation  to  them. 

It  is  interesting  to  observe  in  conclusion  that 
the  parasitic  habit  is  so  largely  developed  among 
the  hymenoptera  that  hyper-parasites — i.e.,  para- 
sites of  parasites — may  reach  even  the  third  and 
fourth  degree. 

Class — Arachnida. — Of  this  class  which  in- 
cludes the  ticks  and  mites,  the  scorpions  and 
spiders,  we  find  practically  all  of  the  ticks  and 
one-half  of  the  mites  to  be  parasitic,  either 
upon  animals  or  upon  plants. 
The  ticks  comprise  two  families,  the  Argasidae 
and  the  Ixodidse  which  form  the  superfamily 


348  BIOLOGY:  GENERAL  AND  MEDICAL 

Ixodiodea.  The  eggs  usually  hatch  upon  the 
ground  yielding  six-legged  "nymphs"  or  larval 
forms.  With  the  second  moult  they  acquire  a 
fourth  pair  of  legs  and  become  mature.  The 
young  ticks  climb  the  stems  of  plants  and  there 
await  the  coming  of  some  warm-blooded  animal 
to  which  they  quickly  transfer  themselves. 
The  proboscides  of  the  little  ticks  cannot  reach 


FIG.  116. — Pyroplasma  bigeminum.  Cause  of  Texas  fever  in  cattle,  in 
stained  blood  of  steer.  X  1000.  a.  Leucocyte;  b,  normal  erythrocyte;  c, 
erythrocyte  containing  one  pair,  d,  erythrocyte  containing  two  pairs  of  pyro- 
plasmata. 

the  blood,  but  their  introduction  into  the  skin 
sets  up  an  inflammatory  reaction  with  some 
edema  and  the  lymph  nourishes  them.  The 
bites  sometimes  suppurate.  When  full-grown 
the  proboscis  easily  penetrates  the  skin,  and  the 
parasites  slowly  distend  themselves  with  blood 
to  a  surprising  extent.  The  male  tick  does 
not  seem  to  be  much  of  a  blood  sucker,  bites 
occasionally,  and  is  satisfied  with  little;  the 


PARASITISM 


349 


female,  however,  inserts  its  proboscis  into  the 
skin  once  for  all  and  attaches  itself  so  firmly  that 
it  cannot  be  pulled  loose  without  tearing  away 
its  mouth  parts.  It  does  not  let  go  until  it  is 
completely  filled  and  has  been  fertilized  by 
the  male  when  it  drops  off  to  lie  helpless  on 
the  ground,  where  after  a  short  time  it  dis- 
charges a  large  mass  of  round  transparent 
eggs  from  which  the  embryo  ticks  hatch. 
Where  ticks  are  numerous  they  may  be  trouble- 
some. They  may  attach  themselves  without 


FIG.  117. — Ornithodorus  moubata.     Tick  that  transmits  African  relapsing  fever: 
a,   Viewed  from  above;  b,  viewed  from  below.     (Robert  Koch.) 


any  sensation  or  disturbance  of  the  host,  or 
their  bites  may  cause  intolerable  irritation. 
Their  chief  importance  depends  upon  the  fact 
that  they  harbor  certain  parasites  which  they 
transmit  from  animal  to  animal,  not  always 
directly,  since  they  rarely  visit  more  than 
one  host,  but  by  taking  the  parasites  from 
the  animal  frequented,  and  passing  them 
through  the  eggs  to  a  new  generation  through 
which  new  animals  become  infected.  It  is  in 
this  manner  that  the  "Texas  fever"  of  cattle,  a 


350 


BIOLOGY:  GENERAL  AND  MEDICAL 


very    dangerous    and    destructive    malady    is 
transmitted.     African  relapsing  fever  is  trans- 
mitted to  man  by  the  African  tick,  Ornitho- 
dorus  moubata,  and  in  the  Rocky  Mountains  a 
febrile  affection,   known  as  bitter  root  fever, 
has  also  been  traced  to  the  bites  of  ticks. 
The  spirillosis  of  fowls  and  ducks  is  spread  by 
the  ticks  (large  lice)  that  infest  them. 
Of  the  Mites  large  numbers  frequent  plants, 


FIG.   118.—  Spirochseta  duttoni.     Rat  blood.      X  1500.     (J/ter 


sucking  the  juices  and  causing  the  leaves  to  dry 
and  curl.  Grain  mites,  that  normally  frequent 
and  undoubtedly  live  upon  the  stems  and  hulls 
of  grains  seem  quite  ready  to  infest  man  when 
opportunity  offers.  Thus  an  extremely  minute 
mite  of  this  kind,  Pediculoides  ventricosus, 
much  too  small  to  be  seen  without  the  aid  of  a 
lens,  and  commonly  found  upon  straw,  some- 
times renders  miserable  the  lives  of  those  that 


PARASITISM  351' 

sleep  upon  straw  beds  by  being  shaken  through 
the  ticking,  penetrating  the  clothing,  and  work- 
ing their  way  into  the  skin  and  causing  severe 
dermatitis.  The  female  mite  introduces  its 
proboscis  into  the  skin,  and  distends  its  body 
to  an  enormous  extent  while  the  surrounding 
tissues  swell  until  an  umbilicated  vesicle  or 
pustule,  not  unlike  that  of  small-pox,  is  formed. 
A  more  familiar  mite  is  the  Acarus  scabiei,  that 
which  causes  the  "  itch  "  or  scabies.  The  female 
bores  tunnels  in  the  epidermis,  causing  hyper- 
emia.  vesication,  and  pustulation  associated 


Fte.  119.— Pedkuloid 

a,  Male;   b. 


with  great  itching.  Scabies  is  by  no  means 
confined  to  man,  but  appears  in  sheep  as  "sheep- 
scab,"  and  in  dogs,  cats,  horses,  and  cattle  as  the 
"mange." 

A  familiar  but  no  less  disagreeable  mite  is  the 
"blackberry  tick"  or  "harvest  bug,"  the  Lep- 
t  us  autumnalis,  an  early  stage  of  a  mite  of  the 
genus  Trombidium,  which,  frequenting  long 
grass  and  blackberry  bushes,  not  infrequently 
finds  its  way  to  the  human  skin  into  which  it 
thrusts  an  enormously  long  proboscis.  Its 
irritating  saliva  causes  considerable  irritation, 


352  BIOLOGY:  GENERAL  AND  MEDICAL 

weal  formation,  and  intense  itching.  When  these 
mites  are  numerous  the  victim  suffers  consider- 
able malaise  and  some  elevation  of  the  body 
temperature.  As  the  weals  form,  the  bodies  of 
the  mites  become  surrounded  by  the  tumefied 
skin  and  appear  as  minute  transparent  red 
points  at  the  centres.  The  life  history  of  these 
mites  is  not  completely  known. 
The  ticks  and  mites  are  not  all  parasitic  upon 


FIG.  120. — Leptus  autumnalis.     (After  Trouessart.) 


warm-blooded  animals;  a  few  infest  snakes, 
reptiles,  and  even  fishes. 

A  peculiar  arachnid,  probably  related  to  the 
mites,  the  Liguatulid,  is  parasitic  upon  the 
tongue  of  dogs. 

Phylum — Chordata. — Of  the  vertebrates  there  are  very 
few  parasitic  representatives. 

Class — Pisces. — The  Hag-fishes  may  with  pro- 
priety be  classed  among  the  parasites.  The 
common  hag,  Myxine  glutinosa,  is  an  eel-like 
fish,  not  infrequently  18  inches  in  length. 


PARASITISM  353 

It  has  a  cartilaginous  skeleton,  eyes  deeply 
embedded  in  the  skin,  and  a  mouth  so  rudi- 
mentary that  it  consists  of  a  mere  membranous 
ring  furnished  with  a  single  tooth  in  its  upper 
part.  The  tongue,  however,  is  furnished  with 
two  rows  of  strong  teeth.  Around  the  mouth 
are  eight  short  tentacles.  With  the  tongue  as 
a  piston  and  using  the  mouth  as  a  sucker,  the 
hag  attaches  itself  to  a  halibut  or  other  large 
fish,  holds  on  firmly  and  gradually  bores  its 
way  into  the  body  cavity,  consuming  the  flesh 
of  the  fish  until  only  the  skin,  entrails,  and 
cartilaginous  skeleton  remain. 

It  is  indispensable  to  successful  parasitic  existence 
that  the  symbiotic  relationship  be  so  adjusted  that 
means  are  provided  for  the  escape  of  the  parasites  or 
their  offspring  in  order  that  new  generations  in  new 
hosts  may  obtain.  With  the  ecto-parasites  this  is  a 
simple  matter,  but  with  those  living  within  the  bodies 
of  the  hosts  it  is  more  difficult. 

The  means  of  transmission  is  very  varied  and  in 
many  cases  very  interesting.  With  the  ocasional 
parasites,  such  as  the  mosquitoes,  fleas,  bed-bugs,  and 
some  of  the  ticks,  the  symbiotic  union  is  so  indefinite 
that  the  host  is  not  fixed.  With  the  lice  which  actually 
live  upon  the  host,  the  transmission  of  the  adult  para- 
sites is  the  accidental  result  of  the  intimate  personal  as- 
sociation of  the  hosts.  In  case  of  such  emergency  arising 
as  the  death  of  the  host,  the  parasites  being  unable  to 
seek  new  hosts,  remain  upon,  and  die  with,  or  rather 
after  him. 

The  intestinal  worms  discharge  enormous  numbers 
of  eggs  which  pass  out  with  the  excrement,  admission  to 
fresh  hosts  being  a  matter  of  chance.  In  such  cases 
it  is  usually  at  the  sacrifice  of  countless  eggs  and  embryos 
that  one  is  preserved  by  entrance  into  a  new  host.  Thus 
the  eggs  falling  upon  the  soil  must  wait  in  some  cases 
until  an  appropriate  animal,  browsing  upon  the  herbage, 

23 


354  BIOLOGY:  GENERAL  AND  MEDICAL 

takes  them  up  with  the  rootlets  of  the  plants.  In  such 
cases  it  may  be  that  the  animal  from  which  the  ova 
come  or  one  of  its  fellows  that  can  act  as  host,  or  it  may 
be  an  entirely  different  kind  of  animal  by  which  the 
egg  must  be  swallowed,  in  order  that  development  may  oc- 
cur. The  intestinal  parasites  of  man  furnish  interesting  ex- 
amples of  direct  and  indirect  infestation.  The  pin-worm 
or  seat-worm  (Oxyuris  vermicularis)  so  common  in  children, 
occasions  considerable  local  irritation  about  the  anus  and 
causes  the  host  to  scratch  the  part,  thus  taking  up  the  eggs 
with  the  nails  and  carrying  them  later  on  to  the  mouth, 
thus  continually  adding  to  the  number  of  his  parasites. 

The  round  worm  of  the  intestine,  Ascaris  lumbricoides, 
lives  in  the  intestines  of  men  and  hogs.  It  discharges 
eggs  surrounded  with  a  thick  albuminous  coating  that 
enables  them  to  resist  drying  for  a  long  time.  These 
eggs  in  the  evacuations  of  the  host,  which  are  frequently 
used  as  fertilizer,  may  adhere  to  green  vegetables,  be 
swallowed,  and  so  reach  the  stomach,  where  the  albuminous 
capsule  is  dissolved  by  the  digestive  juices  and  the  embryo 
set  free.  The  embryo  works  its  way  into  the  circulation, 
is  carried  to  the  lungs,  where  it  seems  to  undergo  an  addi- 
tional development,  "is  coughed  up  and  again  swallowed, 
thus  finally  reaching  the  intestine  ready  to  mature. 

The  hook  worms,  Anchylostoma  duodenale  and  Neca- 
tor  americana,  produce  abundant  eggs  which,  after 
having  been  discharged  in  the  excrement,  develop  in 
moist  soil  into  diminutive  embryos  which  attach  them- 
selves to  the  'skin  of  the  feet  or  hands,  bore  through, 
enter  the  capillaries,  and  are  carried  by  the  blood  to 
the  lungs,  where  they  undergo  a*  further  developmental 
stage,  are  later  coughed  up,  and  some  being  swallowed 
in  the  mucus  find  their  way  to  the  intestine  where  they 
develop  into  the  adult  parasites.  The  eggs  of  Schisto- 
soma  hematobium,  falling  into  water,  develop  into  a  ciliated 
merecidium  or  embryo.  Whether  this  reaches  new  hosts 
by  directly  perforating  the  skin  during  bathing  or  must 
be  swallowed  is  not  yet  known. 

In  all  of  the  examples  thus  far  given  the  transmission 
of  the  parasite  is  said  to  be  direct;  that  is,  from  host  to 


PARASITISM 


355 


host  of  the  same  kind,  the  independent  embryonal  life 
being  quite  short. 

But  in  many  cases  the  embryonal  life  of  the  parasite 
is  so  long  that  a  new  cycle  in  a  second  host  is  required 
to  bring  the  parasite  to  maturity.  Such  conditions 
attend  the  lives  of  the  tape-worms.  The  eggs  of  these 
parasites  leave  the  body  of  the  host  with  his  excrement 
and  presumably  scatter  upon  the  soil.  It  has  been 
found  by  experiment  that  should  any  of  these  eggs  be 
swallowed  by  the  same  host  or  a  host  similar  to  that  in 
whose  body  they  were  produced,  they  sometimes  at  once 


FIG.  121. — Eggs  of  Ticnia  Sagittate. 


FIG.  123.— Head  of  Tsenia  saginata. 
(Mosler  and  Peiper.) 


FIG.  122. — Mature  segments  of 
Taenia  saginata. 


develop  into  the  adult  organism,  but  this  is  by  no  means 
the  rule  and  the  chances  seem  to  be  in  favor  of  their 
being  swallowed  by  some  other  animal  in  whose  body 
they  develop  differently.  Thus  taking  as  an  example 
the  most  common  tape-worm  of  this  country,  Tsenia  sagi- 
nata, the  beef-worm,  it  is  found  that  its  eggs,  presumably 
taken  from  the  soil  by  grazing  cattle,  develop  in  them  to 
an  embryo  in  no  way  similar  either  in  appearance  or 


356 


BIOLOGY:  GENERAL  AND  MEDICAL 


habit  to  its  parent.  Instead  of  growing  into  a  long 
segmented  worm,  the  egg  develops  into  a  short  embryo 
which  undergoes  a  peculiar  cystic  expansion  of  the 
first  segment  into  which  the  head  and  neck  are  withdrawn, 
forming  a  distinct  parasitic  cyst  or  scolex.  Such 
embryos  do  not  inhabit  the  intestine,  but  in  some  way 
leave  that  viscus  to  encyst  themselves  in  the  muscles  of 
the  animal,  take  up  a  kind  of  inactive  existence,  and 
await  the  chance  of  being  eaten  by  man  with  the  flesh 
of  the  ox.  Should  flesh  containing  such  an  embryo  or 
scolex  be  eaten  raw — the  embryo  is,  of  course,  killed  by 
cooking — it  emerges,  as  it  experiences  the  stimulating 


FIG.  124. — Tsenia  saginata.     (Eichhorst.) 

action  of  the  digestive  juices,  thrusts  out  its  head,  attaches 
itself  to  the  intestinal  wall  by  its  cephalic  suckers,  and 
develops  into  an  adult  worm  or  strobila.  We  thus  find 
that  for  parasites  of  this  class  there  are  two  separate 
cycles  of  life,  lived  in  two  different  hosts  in  alternation. 

Of  the  human  parasites  the  intermediate  hosts  are 
numerous  and  varied;  thus  for  Taenia  saginata  it  is  the 
bovine  species;  of  Tasnia  solium,  the  hog;  of  Dibothrio- 
cephalus  latus,  the  pike;  of  Paragonimus  westermanii, 
a  mollusk. 

Man  is  himself  the  intermediate  host  of  Taenia  echino- 
coccus,  the  adult  of  which  infests  the  dog. 

Blood  parasites,   shut  in  the  circulatory  apparatus; 


PARASITISM 


357 


were  for  a  long  time  a  source  of  much  perplexity  as  no 
means  for  transmission  from  animal  to  animal  could  be 
found.  It  was  agreed  on  all  sides  that  the  malarial 
parasites  did  not  leave  the  body  in  the  expired  air, 
in  the  urine,  or  in  the  feces;  the  embryos  of  Filaria  ban- 
crofti  (Filaria  sanguinis  hominis)  were  in  the  blood  in 
large  numbers,  but  could  not  be  traced  from  the  body, 
their  occasional  appearance  in  the  urine  being  undoubt- 
edly accidental.  It  was  the  genius  of  Sir  Patrick  Manson 
that  afforded  the  first  clue.  Finding  that  the  embryos  of 
Filaria  bancrofti  appeared  in  the  blood  at  night,  he 
conjectured  that  it  might  be  to  adapt  them  to  the  visits 


FIG.  125.— Filaria  embryo,  alive  in  the  blood.     (F.  P.  Henry.) 


of  some  nocturnal  blood-sucking  insect.  Working 
upon  this  hypothesis,  he  and  Low  found  that  when  a 
common  mosquito,  Culex  pipiens,  draws  blood  containing 
these  worms  into  its  stomach,  the  embryos  shed  their 
hyaline  sheaths,  bore  through  the  intestinal  wall,  migrate 
to  the  thoracic  muscles,  and  encyst  themselves  at  the 
base  of  the  proboscis.  After  a  period  of  rest,  the  worms, 
feeling  the  stimulating  effects  of  warm  blood  as  the 
mosquito  bites  again,  leave  the  muscles,  enter  the  pro- 
boscis, and  work  their  way  into  the  tissue  of  the  newly 
bitten  host. 

The    further   history    is    uncertain;    presumably    the 
embryos  at  once  take  up  their  habitat  in  the  lymphatic 


358  BIOLOGY:  GENERAL  AND  MEDICAL 

system,  grow  to  maturity,  and  later  fill  the  blood  with 
their  own  embryo-descendants. 

This  discovery  led  Manson  to  suspect  that  the  malarial 
parasite  might  have  a  similar  intermediate,  and  experi- 
ments with  mosquitoes  showed  him  that  when  blood 
containing  malarial  parasites  was  taken  into  the  mos- 
quito's stomach-intestine,  the  parasites  underwent  a 
change  known  as  flagellation,  which  was  suspected  to  be 
the  beginning  of  a  new  life  cycle.  Manson  was  not  able  to 
perfect  this  work,  but  his  pupil,  Sir  Ronald  Ross,  acting 
upon  his  suggestions,  worked  patiently  upon  the  problem 
in  India  and  succeeded  in  showing  that  certain  mosquitoes, 
the  genus  Anopheles,  do  act  as  hosts  of  the  parasites. 


FIG.  126. — Filarial  worm  (a)  in  proboscis  of  Culex  pipiens. 

The  last  step  in  a  complete  understanding  of  the  life 
history  of  the  parasites  was  made  by  MacCallum  in 
this  country. 

In  brief,  the  life  history  is  as  follows:  The  parasites 
live  one  cycle  in  the  body  of  man,  where  they  first  appear 
as  minute  amreboid  bodies  in  the  red  blood  corpuscles. 
These  grow  and  destroy  the  corpuscles  until  they  attain 
to  an  almost  equal  size,  when  they  divide  into  a  varying 
number  of  small  bodies  or  spores  (the  parasite  belongs 
to  the  class  Sporozoa  of  the  Phylum  Protozoa).  These 
spores  at  once  attach  themselves  to  other  corpuscles  and 
repeat  the  phenomena  of  growth  and  sporulation  and  so 
on,  a  new  crop  maturing  every  third  or  fourth  day 


PARASITISM 


359 


according  to  the  species  of  the  parasite.  When  the 
number  of  parasites  in  the  blood  becomes  considerable, 
a  variation  presents  itself  in  that  some  of  adult  parasites 
that  do  not  sporulate,  but  remain  large,  ovoid  or  cres- 
centic  bodies,  the  gametes,  or  sexually  perfect  parasites. 
When  the  blood  is  drawn,  either  for  microscopic  exami- 
nation in  the  fresh  state,  or  by  the  mosquito,  a  change 
takes  place  in  certain  of  these  bodies  which  become 
actively  amoeboid,  show  tumultuous  cytoplasmic  stream- 


FIG.  127. — Parasite  of  tertian  malarial  fever,  a,  b,  c,  d,  e,  f,  g,  Growing 
pigmented  parasite  in  the  red  blood  corpuscles;  h,  spores  formed  by  segmentation 
of  the  parasite — no  roset  is  found,  but  concentric  rings  of  the  cytoplasm  divide: 
i,  macrogametocyte;  /,  microgametocyte  with  flagella  or  spermatozoits. 

ing,  and  then  emit  delicate  filamentous  bodies  of  minute 
size  formerly  called  "flagella."  It  was  supposed  by 
Manson  that  these  were  the  essential  parasitic  forms  of 
the  mosquito  cycle,  but  MacCallum  showed  that  they 
are  the  spermatozoids  of  the  parasite,  and  he  was  able 
to  follow  them  to  the  female  gametocytes  or  oocytes  and 
actually  observed  the  process  of  fertilization  in  the  case 
of  an  avian  malarial  parasite  known  as  Plasmodiumi 
(Halteridium)  danelewskyi. 
When  fertilization  has  taken  place  in  the  mosquito's 


360 


BIOLOGY:  GENERAL  AND  MEDICAL 


body,  a  vermicular  zygote  results,  which  penetrates  the 
intestinal  wall  and  remains  attached  to  its  outer  surface, 
projecting  into  the  abdominal  cavity.  The  zygote 
enlarges,  breaks  up  into  numerous  zygomeres,  each 


FIG.  128.— Developmental  cycle  of  the  malarial  parasite  in  the  mosquito. 
a,  b,  c,  Zygocytes;  d,  e,  blastomeres  with  contained  sporozoits;  /,  sporozoits  mi- 
grating into  the  salivary  gland-cells.  (From  Manson.) 


of  which  eventually  divides  into  a  large  number  of 
sporozoits  of  falciform  shape  which  migrate  to  the 
cells  of  the  salivary  glands,  from  which  they  enter  the 
insect's  saliva.  This  transformation  requires  from  ten 


Fia.  129. — Stomach  of  mosquito  with  zygocytes  on  the  outer  surface. 


to  fourteen  days,  according  to  the  temperature.  When 
the  mosquito  subsequently  bites,  the  sporozoits  entering 
the  blood  of  the  new  individual  attach  themselves  to 
the  red  corpuscles,  and  again  begin  the  human  cycle. 


PARASITISM 


361 


Thus,  in  each  of  these  cases  the  parasite  lives  alter- 
nately in  two  hosts,  one  of  which,  the  insect,  acts  as  the 
distributor. 

These  discoveries  gave  a  great  impetus  to  the  investi- 
gation of  the  means  by  which  other  blood  parasites  were 
transmitted,  and  it  is  now  known  that  the  trypanosomes 
of  rats  are  transmitted  by  their  insect  parasites,  the 
trypanosomes  of  Nagana  by  the  "tsetse  fly,"  Glossina 
morsitans,  and  the  trypanosomes  of  African  lethargy  by 


FIG.    130. — Glossina  palpalis    (X    3   3/4),   the  carrier  of   the  trypanosome  of 
sleeping  sickness.     (After  Adami.) 

the  tsetse  flies,  Glossina  palpalis  and  Glossina  morsitans. 
It  has  also  been  discovered  that  the  trypanosomes  of  Surra 
and  of  mal  de  caderas  are  transmitted  by  a  biting  fly  fairly 
well  identified  as  Stomoxys  calcitrans;  and  that  the  spiro- 
cha3te  of  African  relapsing  fever  is  transmitted  by  a  tick, 
the  Ornithodorus  moubata. 

The  mention  of  the  tick  introduces  another  matter 
of  interest,  for  it  is  not  the  tick  that  sucks  the  blood  that 
transmits  the  disease,  but  its  progeny,  which  have  been 
infected  as  eggs  in  the  body  of  the  mother. 


362 


BIOLOGY:  GENERAL  AND  MEDICAL 


The  spirochetes  have  actually  been  seen  in  the  mother's 
body  in  the  ovaries,  and  enter  the  eggs  themselves  where 
they  appear  as  congeries  of  fine  granules. 

These  observations  make  clear  the  transmission  of 
Texas  fever  by  the  Ripicephalus  bovis.  The  female 
tick  distended  with  the  infectious  blood  drops  off,  ovipos- 
its, and  dies,  but  the  immature  ticks  transmit  the  disease 
to  cattle  so  soon  as  they  reach  them.  The  explanation  is 
found  in  the  passage  of  the  parasites  into  the  egg  and  the 
infection  of  the  embryo. 


FIG.  131. — Spirochsete  obermeieri.     (Novy.)     Rat  blood.    X  1500. 

The  Leptospira  icteroides  now  thought  to  be  the  micro- 
parasite  of  yellow  fever,  is  harbored  and  transmitted  by 
the  mosquito  Aedes  (Stegomyia)  calopus. 

In  considering  the  reciprocal  relations  of  the  parasites 
and  their  hosts  it  is  important  to  remember  that  the  host 
is  necessary  to  the  parasite  and  that  his  untimely  death 
may  interrupt  one  of  the  developmental  cycles  by  which 
the  permanence  of  the  parasite  is  secured.  The  con- 
tinuance of  the  parasitic  relationship  may  therefore 


PARASITISM  363 

be  referred  to  the  circumstance  that  the  injury  to  the  host 
ought  not  be  so  great  as  to  prevent  the  completion  of 
at  least  one  generation  of  the  parasites,  or  the  preparation 
of  one  crop  of  eggs  or  embryos  for  future  activity.  The 
waste  in  parasite  eggs  and  embryos  is  immense;  enor- 
mous numbers  are  produced,  few  survive. 

REFEEENCES. 

R.  LEUCKART:  "  Die  Parasiten  des  Menschen, "  etc.,  Leipzig,  1881. 
M.  BRAUN:  "Tierische  Parasiten  des  Menschen,"  Wiirzburg, 

1908. 

The  New  International  Encyclopedia:  Article  upon  "Parasites." 
JOSEPH  MCFARLAND:  "Text-book  on  Pathology,"  Phila.,  1910, 

Chapter  upon  Parasitism. 


CHAPTER   XV. 
INFECTION  AND  IMMUNITY. 

When  parasitic  symbionts  are  fairly  large  they  are 
said  to  infest  the  host;  when  very  small,  to  infect  it. 
Between  infestation  and  infection  no  sharp  distinction 
can  be  drawn,  though  it  is  probably  more  correct  usage 
to  employ  the  term  "infection"  for  the  invasion  of  the 
body  by  microparasites  of  any  kind.  Any  object  upon 
which  such  organisms  may  be  brought  into  the  body  is 
said  to  be  infective.  The  body  into  which  they  are 
brought  is  said  to  be  infected,  the  organisms  through 
which  the  infection  is  brought  about  are  said  to  be 
infectious,  Infection,  being  a  form  of  parasitism, 
implies  injury  of  the  host  by  the  microparasites. 

Infecting  organisms  are,  therefore,  always  pathogenic, 
or  capable  of  exciting  anatomical  or  physiological  dis- 
turbances. The  injurious  quality  of  the  organism  is 
characterized  as  its  virulence,  and  depends  upon  con- 
ditions attending  its  metabolism.  Thus,  it  may  liber- 
ate enzymes,  it  may  produce  toxic  proteins,  it  may 
transform  the  chemical  reaction  of  the  surrounding 
media,  or  it  may  facilitate  its  own  invasive  powers  by 
giving  off  certain  offensive  substances  (aggressins)  by 
which  the  defensive  reactions  of  the  host  are  inhibited 
in  action. 

The  almost  universal  prevalence  of  bacteria  deter- 
mines that  no  higher  organism  escapes  contact  with 
them.  Through  how  wide  a  range  their  power  of  in- 
vading the  tissues  of  other  living  things  may  extend  is 
difficult  to  answer;  it  is  doubtless  very  wide,  and 
includes  both  plants  and  animals. 

364 


INFECTION    AND    IMMUNITY  365 

Certain  microorganisms  are  invariable  commensals 
of  the  higher  organisms,  frequenting  various  surfaces 
of  the  body  upon  which  or  in  which  the  conditions  of 
life  are  favorable  to  them.  When  accident  determines 
that  such  shall  be  carried  into  the  tissues,  they  may  or 
may  not  be  able  to  survive  the  change.  In  most  cases 
they  quickly  succumb  to  the  unusual  conditions.  In 
other  cases  they  survive  for  a  limited  time,  during 
which  the  host  suffers  more  or  less  disturbance,  and 
after  which  their  vitality  wanes,  they  die  out  and  the 
damage  they  have  effected  may  be  repaired.  Far  more 
injurious  are  the  new  and  strange  microparasites  with 
which  one  occasionally  comes  into  contact.  Some 
of  these  are  actively  invasive,  find  their  way  from  the 
skin,  the  respiratory  or  the  alimentary  organs  into  the 
blood,  distribute  throughout  the  body,  sometimes 
exciting  local  histological  changes  in  the  tissues,  some- 
times general  physiological  disturbances,  as  fever,  etc. 
Such  organisms  may  quickly  or  slowly  destroy  the  host, 
though  in  perhaps  a  majority  of  the  cases  they,  too, 
become  less  vigorous  as  time  goes  on,  and  die  out,  leav- 
ing the  patient  to  recover  if  not  too  much  damaged  by 
their  inroads. 

All  grades  of  invasiveness  occur.  Some  micro- 
parasites  find  a  very  superficial  and  restricted  field  of 
operation;  some  penetrate  more  deeply  and  are  carried 
in  small  numbers  in  the  lymph  and  blood  vessels  to  the 
viscera  where  they  slowly  occasion  minute  changes,  and 
still  others  freely  distribute  through  the  blood  and 
affect  the  entire  constitution  of  the  host. 

The  products  of  the  microparasites  must  not  be 
neglected  when  considering  their  pathogenesis;  they  pass 
through  all  grades  of  harmfulness.  Some  microorganisms, 
though  they  invade  easily,  produce  only  a  mild  fever; 
others  incapable  of  extensively  invading  the  body 
of  the  host,  form  poisonous  products  which  when  ab- 
sorbed into  the  blood  affect  tissues  remote  from  the  seat 
of  microparasitic  invasion.  This  is  well  exemplified  by 


366  BIOLOGY:  GENERAL  AND  MEDICAL 

the  bacillus  of  tetanus  which,  though  unable  to  invade 
the  body  generally,  eliminates  a  soluble  toxin  that  usually 
causes  the  death  of  the  host  by  its  exciting  action  upon 
the  nervous  system. 

In  order  that  infection  be  possible,  certain  conditions 
must  be  fulfilled.  These,  which  may  be  described  as  the 
cardinal  conditions  of  infection  are:  1.  The  infecting 
organism  must  enter  the  host  in  sufficient  number;  2. 
It  must  enter  by  an  appropriate  avenue;  3.  It  must  be 
virulent;  4.  The  host  must  be  receptive. 

Among  such  delicately  organized  bodies  as  the  micro- 
parasites  the  tenure  of  life  is  short,  and  any  radical  change 
is  apt  to  be  accompanied  by  a  high  mortality.  When 
we  endeavor  to  determine  the  actual  number  of  micro- 
organisms, in  any  culture,  requisite  to  infect,  we  usually 
find  that  not  a  few  of  the  counted  and  estimated  organ- 
isms are  already  dead.  Infectivity  is,  moreover,  a 
quality  not  possessed  by  all  the  organisms  in  equal  de- 
gree. Some  are  more  infective  than  others,  so  that  it  is 
usually  necessary  for  a  considerable  number  to  enter  the 
host  in  order  that  sufficiently  virulent  organisms  may 
survive  the  change  and  effect  the  invasion. 

The  number  of  organisms  naturally  bears  some  relation 
to  their  virulence  or  injurious  power.  If  they  are  of 
slight  virulence  a  much  greater  number  may  be  required 
than  when  they  are  highly  virulent.  Virulence  is  a 
variable  quality,  comparable  to  the  color  or  perfume  of 
flowers,  and,  like  them,  subject  to  modification  through 
circumstance.  It  is  not  known  whether  mild  virulence 
signifies  that  all  of  the  organisms  in  a  culture  are  uni- 
formly weak  in  this  particular  or  that  many  or  most  of 
them  are.  Presumably  it  is  the  latter,  for  when  the 
culture  is  experimentally  placed  under  conditions 
favorable  to  virulence,  it  sometimes  speedily  revives. 
Thus,  if  bacteria  are  transplanted  many  times  upon 
artificial  culture  media  they  lose,  but  if  they  are  passed 
through  a  succession  of  animals  for  which  they  are 
invasive,  they  increase  in  virulence.  This  makes  it 


INFECTION   AND    IMMUNITY  367 

appear  as  though  among  the  individual  bacteria  com- 
prising the  culture  some  were  pathogenic  and  some 
vegetative.  When  the  organisms  are  frequently  trans- 
planted from  culture  medium  to  culture  medium,  the 
vegetative  individuals  thrive  best  and  eventually  alone 
survive,  the  others  having  been  outgrown  and  outlived, 
after  which  no  virulence  can  be  revived.  When,  on  the 
other  hand,  they  are  frequently  passed  through  animals 
the  pathogenic  individuals  thrive  and  the  vegetative 
ones  are  eliminated. 

The  avenue  by  which  the  microparasites  enter  the  host 
seems  to  be  of  importance.  Certain  species  thrive  only 
when  taken  into  the  alimentary  organs;  others  only 
when  applied  to  the  mucous  membranes;  still  others 
only  when  taken  into  the  respiratory  organs.  The 
avenue  of  entrance  also  determines  the  form  that  an 
infection  may  take.  Thus  streptococci,  minute  spheri- 
cal organisms,  hanging  together  like  a  string  of  beads, 
may  cause  erysipelas  if  entering  through  the  skin;  sore 
throat  with  the  formation  of  a  false  membrane  if  taken 
into  the  mouth;  abscesses  if  carried  into  the  deeper 
tissues;  puerperal  fever  if  introduced  into  the  uterus,  and 
disease  of  the  valves  of  the  heart  if  into  the  circulation. 

The  host  must  be  in  a  receptive  condition.  This  is  one 
of  the  most  interesting  of  all  the  cardinal  conditions, 
implying  as  it  does  some  variation  in  the  physiologico- 
chemical  condition  of  the  host  by  which  the  invasive 
power  of  the  microparasites  is  made  possible  or  im- 
possible. When  the  host  is  receptive  it  is  described  as 
susceptible;  when  resistant,  as  immune. 

Immunity  may  be  denned  as  a  physiologico-chemical 
condition  of  the  host  by  which  its  invasion  by  micro- 
parasites  is  made  impossible.  When  one  inquires  the 
nature  of  this  condition,  he  enters  upon  an  investigation 
the  scope  and  complexity  t>f  which  seem  to  increase  as  the 
subject  is  pursued.  Microparasites  eliminate  "aggressins" 
tending  to  destroy  the  body  defenses,  toxins  by  which  the 


368  BIOLOGY:  GENERAL  AND  MEDICAL 

cells  of  the  host  are  poisoned,  enzymes  by  which  they  are 
dissolved,  and  alter  the  chemical  reaction  of  the  tissues  in 
which  they  arrive.  Before  the  parasite  destroying  mecha- 
nism can  be  set  in  operation,  therefore,  something  in  the  way 
of  endurance,  tolerance,  or  immunity  against  the  micro- 
parasitic  products  becomes  essential.  The  reactions  of 
immunity,  therefore,  become  exceedingly  complex.  They 
embrace  two  essentials — ability  to  endure  the  microorgan- 
ismal  products  without  injury  and  ability  to  destroy  the 
microparasites. 

Before  further  pursuing  the  subject  of  infection,  a 
brief  space  must  be  devoted  to  general  considerations 
that  bear  upon  the  subject  of  immunity. 

Habit  has  much  to  do  with  the  general  vital  processes. 
It  is  exemplified  by  the  change  of  animals  from  the 
independent  and  synthetic  to  the  dependent  and  analytic 
mode  of  nutrition  and  by  the  remarkable  differences  in 
the  behavior  of  aquatic  animals  to  salt  and  fresh  water. 
The  great  marine  mammals,  the  fishes,  the  numerous 
phyla  of  marine  invertebrates,  and  large  numbers  of 
marine  birds  find  sea-water  with  its  heavy  percentage 
of  sodium  chloride  and  other  salts  unobjectionable; 
other  animals  and  plants  habituated  to  fresh  water  are 
poisoned  by  it;  still  other  animals  and  plants  living  in 
the  mouths  of  rivers  endure  alternating  immersion  in 
salt  and  fresh  water  resulting  from  tides  and  currents 
without  injury.  The  precisely  arranged  adaptations 
under  which  living  things  present  themselves  to  obser- 
vation at  the  present  time  are  the  result  of  divergences 
begun  long  ago,  and  fail  to  suggest  the  tremendous 
sacrifice  of  life  though  which  they  have  probably  been 
adjusted. 

Every  creature  is  found  upon  examination  not  only 
to  consume  by  preference  some  particular  kind  of  food, 
but  to  find  radical  departure  from  it  fatal  and  even 
slight  departures  harmful.  Even  where  the  diet  appears 
to  be  of  mixed  nature  it  is  limited  to  a  few,  not  to  all 


INFECTION   AND   IMMUNITY  369 

available  foods.  Should  one  inquire  how  and  why  this 
began,  the  question  leads  inevitably  into  the  consideration 
of  the  greater  problems  of  evolution,  for  it  is  clear  that 
such  conditions  cannot  always  have  obtained,  seeing 
that  there  must  have  been  a  time  when  neither  the 
living  thing  nor  its  food  was  in  existence.  That  which 
is  consumed  must  have  come  into  existence  before  that 
which  consumes  it.  It  can  be  shown  that  the  food  habit 
of  existing  animals  sometimes  undergoes  considerable 
modification.  Thus  the  Colorado  potato  beetle,  in  its 
native  habitat,  Colorado,  fed  upon  certain  plants  of  the 
genus  Solanum.  These  plants  were  not  very  common, 
and  the  beetles  were  not  very  numerous  until  civiliza- 
tion arrived  and  they,  by  preference,  seized  upon  the 
potato,  a  related  plant,  and  have  become  a  great  nui- 
sance to  the  farmer.  Perhaps  it  is  not  unusual  for 
animals  to  try  a  new  means  of  subsistence;  if  so,  we  can 
only  know  about  it  when  they  are  successful,  for  in  case 
of  failure  they  would  die  and  escape  observation. 

When  experimental  endeavors  are  made  to  accustom 
animals  to  new  kinds  of  food,  they  commonly  sicken 
and  may  die.  It  is  well-known  that  many  things  are 
distinctly  poisonous  for  the  higher  animals.  What 
is  it  that  determines  the  poisonous  or  non-poisonous 
nature  of  the  food? 

There  is  scarcely  an  organic  poison  known  upon  which 
some  living  thing  may  not  feed  with  impunity.  Tobacco, 
for  example,  is  intensely  poisonous  to  most  mammals, 
birds,  insects,  and  arachnids,  yet  the  growing  plant  is  so 
constantly  attacked  by  the  caterpillar  of  a  large  moth 
that  the  farmer  must  look  at  his  plants  every  day  to  see 
that  the  leaves  are  not  eaten  and  made  unmarketable. 
The  dried  leaves  lacking  the  water  of  the  growing  plant 
contain  a  relatively  greater  proportion  of  the  poison,  yet 
constitute  the  regular  food  of  the  larva  of  a  beetle. 
Manufactured  tobacco  and  cigars  are  not  infrequently 
ruined  by  being  drilled  with  holes  by  them.  How  do 
these  insects  escape  injury,  grow  and  thrive  upon  a 

24 


370  BIOLOGY:  GENERAL  AND  MEDICAL 

plant  whose  juices  furnish  one  of  the  best  insecticides 
known?  Can  it  be  referred  to  habit?  Are  we  justified 
in  supposing  that  at  some  time  when  both  insects  and 
plants  were  evolving  toward  the  forms  in  which  we  now 
know  them,  and  the  plant  perhaps  contained  less  nico- 
tine, a  symbiosis  was  formed  which,  continuing  until  the 
present  time,  affords  these  interesting  examples  of  free- 
dom from  intoxication?  It  would  be  of  interest  if  this 
were  so,  but  it  may  be  an  entirely  erroneous  assumption, 
for  the  insects  may  suddenly  have  taken  to  the  tobacco 
plants  and  for  other  reasons  have  remained  unharmed. 

In  considering  these  topics  we  must  not  forget  that  even 
lowly  animals  are  provided  with  organs  of  special  sense 
and  that  disagreeable  impressions  received  from  other- 
wise desirably  foods  may  keep  them  away.  Upon  such 
grounds  may  the  careful  avoidance  of  certain  apparently 
useful  foods  be  partly  accounted  for.  It  may  have  been 
that  at  some  period  of  famine,  the  repugnance  to  the  odor 
or  taste  giving  place  to  necessity,  the  caterpillar  of  the 
sphinx  was  driven  to  the  tobacco  as  the  only  available 
food,  to  which  it  became  accustomed  and  upon  which  it 
has  remained. 

If  it  could  be  shown  that  habituation  was  able  to 
effect  the  tolerance  shown  to  the  poisons  upon  which 
certain  animals  feed  and  was  at  the  foundation  of  the 
physiologico-chemical  difference  between  them  and 
other  animals  not  possessing  such  tolerance,  it  would 
aid  us  in  our  study  of  the  problems  of  immunity  and 
infection.  Habituation  plays  a  large  part  in  what  is 
called  acquired  immunity.  Need  the  reader  be  re- 
minded that  men  commonly  habituate  themselves  to  to- 
,  bacco  and  opium  and  acquire  a  tolerance  to  these  poisons 
far  beyond  that  generally  shared  by  their  kind? 

Unfortunately,  the  phenomena  of  infection  and  im- 
munity are  incapable  of  reduction  to  general  principles 
in  the  present  state  of  knowledge. 

In  looking  over  the  field  we  first  find  the  condition 
known  as  Natural  Immunity  in  which,  for  no  reason  that 


INFECTION   AND   IMMUNITY  371 

can  be  determined,  certain  animals  are  exempt  from 
the  injurious  effects  of  poisons  or  from  invasion  by  cer- 
tain microparasites. 

Remembering  the  suggestions  concerning  the  habit- 
uation  of  animals  to  poisonous  foods  and  the  immu- 
nity they  seem  to  enjoy  in  consequence,  we  turn  to  an- 
other aspect  of  the  problem. 

Are  microorganisms  subject  to  the  same  ill  effects 
experienced  by  the  higher  organisms  when  the  quality 
of  their  nourishment  is  altered?  Can  this  be  the  expla- 
nation of  the  rapidity  with  which  they  invade  the  body 
of  one  host  and  die  out  in  that  of  another?  It  seems 
justifiable  to  apply  the  same  method  to  both,  and  by 
doing  so  their  behavior  under  certain  conditions  will  be 
explained. 

It  is  much  more  satisfactory  to  consider  the  subjects 
immunity  and  infection  with  reference  to  the  host, 
though  to  lose  sight  of  the  microparasite  and  of  the  recip- 
rocal relations  of  host  and  parasite  will  be  to  lose  much 
that  is  of  fundamental  importance.  Indeed,  almost 
everything  that  is  said  of  one  applies  with  equal 
force  to  the  other. 

Experiments  with  the  bacteria  have  shown  that  they 
quickly  accustom  themselves  to  certain  hosts.  Thus, 
streptococci  by  special  manipulations  may  be  made  so 
virulent  for  rabbits  that  a  mathematical  calculation 
may  show  that  a  single  coccus  may  be  fatal.  At  the 
same  time,  however,  they  may  not  increase  in  virulence 
for  other  kinds  of  animals.  Not  only  do  they  become 
habituated  to  a  certain  kind  of  animal,  but  there  is 
evidence  to  show  that  they  may  even  become  habituated 
to  some  particular  organ  of  the  animal;  thus,  streptococci 
taken  from  a  lesion  of  the  kidney  of  an  animal  and  in- 
jected into  the  circulation  of  a  new  animal  of  the  same 
kind  are  said  to  colonize  in  greater  numbers  in  the 
kidneys  than  elsewhere  in  the  body. 

.  Here  we  seem  to  have  definite,  though  not  necessarily 
fixed  results  following  habituation. 


372  BIOLOGY:  GENERAL  AND  MEDICAL 

Taking  a  preview  of  the  field  of  natural  immunity, 
we  find  that  there  are  many  instances  of  tolerance  to 
poisons.  Thus  in  addition  to  such  cases  as  the  tobacco 
worm  and  its  vegetable  food,  we  find  the  mongoose  and 
hedgehog  fairly  immune  to  the  venoms  of  the  serpents 
they  kill  and  eat.  If  we  take  care  for  study  the  toxins 
separated  by  filtration  from  cultures  of  certain  bacteria, 
we  find  that  the  rat  is  immune  against  diphtheria  toxin 
and  the  hen  against  tetanus  toxin. 

Here  the  limits  of  the  habituation  theory  are  exceeded, 
for  it  is  as  impossible  to  connect  the  rat  with  diphtheria 
and  the  hen  with  tetanus  as  it  is  easy  to  connect  the 
mongoose  and  hedgehogs  with  venom. 

In  regard  to  infection,  the  same  general  facts  are  true. 
There  are  certain  infections  of  plants  not  known  among 
animals;  there  are  certain  infections  peculiar  to  the 
lower  animals — hog  cholera,  swine-plague,  chicken 
cholera,  mouse  septicaemia,  quarter  evil,  etc. — against 
all  of  which  human  beings  are  immune;  there  are  certain 
diseases  of  man — scarlatina,  varicella,  whooping-cough, 
yellow  fever,  etc. — against  all  of  which  the  lower  animals 
are  immune;  and  there  are  certain  diseases — anthrax, 
tuberculosis,  glanders,  actinomycosis,  etc. — to  which 
both  man  and  the  lower  animals  are  susceptible. 

We  are  in  the  habit  of  speaking  of  certain  diseases 
as  peculiar  to  certain  animals,  which  of  course  means 
that  other  animals  are  immune.  Thus  if  one  speaks 
of  glanders,  the  horse  and  ass  are  at  once  thought  of 
and  the  possibility  of  human  infection  may  be  considered, 
but  no  one  thinks  of  cattle,  sheep,  dogs,  or  fowls  because 
it  is  well  known  that  they  are  immune. 

In  all  cases  of  such  natural  immunity,  whether  it  be 
shown  by  exemption  from  the  ill  effects  of  toxins  or  by 
exemption  from  invasion  by  microparasites,  we  find  it 
common  to  all  of  the  animals  of  the  kind.  It  is  an  ex- 
emption of  a  whole  group,  not  of  an  individual,  and  it 
bespeaks  some  kind  of  physiologico-chemical  peculiarity 
antagonistic  to  the  particular  microparasites  against 
which  they  are  immune. 


INFECTION   AND   IMMUNITY  373 

It  is  a  mistake  to  think  of  immunity  with  reference 
to  physical  similarity  or  dissimilarity.  The  glanders 
bacillus  finds  the  white  mouse  immune,  but  the  field 
mouse  the  most  susceptible  of  all  animals.  The  differ- 
ences between  these  two  species  of  mice  do  not  appear 
great.  It  is  also  impossible  to  connect  the  immunity  of 
the  white  mouse  with  any  habituation  it  or  its  ancestors 
may  have  enjoyed. 

It  is  difficult  to  make  generalizations  concerning 
natural  immunity  because  of  its  relative  character. 
What  do  we  mean  when  we  say  that  an  animal  is  immune? 
The  rat  has  been  declared  immune  against  diphtheria; 
it  resists  infection,  is  not  injured  by  several  thousand 
times  as  much  filtered  culture  (toxin)  as  will  kill  a  guinea- 
pig,  but  may  be  killed  if  the  quantity  of  toxin  injected 
into  it  be  out  of  all  proportion.  The  case  of  the  hen  is 
similar;  it  resists  infection  with  the  tetanus  bacillus, 
resists  the  injurious  effects  of  reasonable  amounts  of  its 
toxin,  but  may  be  killed  if  the  quantity  of  toxin  injected 
into  it  be  extreme.  The  mongoose  suffers  but  little 
from  a  snake  bite  such  as  would  destroy  a  rabbit  and 
is  therefore  said  to  be  immune  against  venom  but  it 
may  be  killed  if  overwhelmed  by  the  poison.  Thus  we 
see  that  the  tolerance  is  in  many  cases  relative  and  not 
absolute. 

In  Acquired  Immunity  we  have  to  do  with  an  ability  to 
endure  intoxication  or  to  resist  infection  that  has  been 
acquired  by  an  individual  of  a  naturally  susceptible 
kind.  It  is  a  peculiarity  of  the  individual  not  shared 
by  his  kind.  It  is  acquired  through  circumstances  arising 
during  his  own  lifetime,  and  is  neither  inherited  from 
his  antecedents  nor  transmitted  to  his  descendants. 

Numerous  examples  of  acquired  immunity  occur  in 
the  experience  of  most  persons.  In  childhood  most  of 
us  pass  through  the  throes  of  measles,  chicken-pox, 
whooping-cough,  •  scarlatina  and  mumps,  and  these 
diseases  we  do  not  expect  to  have  again,  because  they 
usually  leave  acquired  immunity  that  persists  through- 


374  BIOLOGY:  GENERAL  AND  MEDICAL 

out  an  ordinary  lifetime.  Our  parents  probably  suf- 
fered from  the  same  affections,  but  we  did  not  profit  by 
their  sufferings,  and  our  children  shall  not  profit  by  ours. 
Most  of  us  have  been  vaccinated  and  thereby  acquired 
immunity  against  small-pox,  but  do  not  transmit  it 
to  our  descendants. 

Acquired  immunity,  therefore,  appears  to  be  some- 
thing within  the  province  of  experiment  and  about 
which  much  may  be  learned.  It  is  also  a  practical 
matter,  for  means  by  which  immunity  against  the  infec- 
tious diseases  may  be  acquired  and  these  diseases  pre- 
vented is  of  the  utmost  importance  to  society. 

By  what  means  can  immunity  be  acquired?  The 
answer  to  this  question  is,  by  habituation.  In  discussing 
the  elementary  characteristics  of  living  matter  it  was 
shown  that  reaction  to  stimulation  becomes  less  pro- 
nounced the  more  frequently  the  stimulations  are 
repeated,  provided  such  stimulations  are  not  of  an  inten- 
sity injurious  to  the  protoplasm  or  produced  by  stimuli 
destructive  in  quality.  In  many  cases  the  failure  of  the 
irritable  response  is  due  to  fatigue  or  exhaustion;  in 
other  cases  it  may  be  due  to  habituation.  That  is, 
the  stimulant  having  proven  less  harmful  than  at  first 
appeared,  an  adjustment  is  effected  by  which  subse- 
quent contacts  produce  diminishing  effects.  This  habit- 
uation of  the  cell  to  repeated  stimulation  and  the  con- 
sequent diminution  of  the  reaction  may  be  fundamental 
in  explaining  acquired  immunity. 

The  phenomena  of  immunity  embrace  two  different 
series  of  reactions,  one  of  which  is  directly  referable  to 
the  cells,  which  participate  actively,  the  other  indirectly 
referable  to  the  cells.  The  former  are  directed  toward 
the  destruction  of  the  organized  living  entities — micro- 
parasites, — the  latter  toward  the  destruction  of  their 
chemical  poisons  (toxins). 

Let  us  first  consider  the  reactions  of  intoxication. 
It  is  well  known  that  by  frequent  administration  and 
cautious  increase  in  the  dosage,  tolerance  can  be  estab- 


INFECTION    AND    IMMUNITY  375 

lished  to  certain  mineral  poisons,  such  as  arsenic,  and  to 
many  alkaloids,  such  as  morphine,  strychnine,  and  nico- 
tine. No  other  explanation  is  at  hand  than  that  this 
depends  upon  the  general  principle  that  habituation  to 
the  poison  diminishes  the  tendency  of  the  protoplasm 
to  become  influenced  by  it.  In  these  cases  the  toler- 
ance is  usually  very  limited,  and  any  sudden  increase  in 
the  dosage  may  be  followed  by  death.  The  phenomena 
attending  such  tolerance  are  essentially  dissimilar  from 
those  following  habituation  to  the  microorganismal 
toxins,  which  are  chemically  different,  being  colloidal 
and  protein  in  nature. 

It  is  not  improbable  that  the  different  reactions  of 
the  organism  toward  the  protein  poisons,  the  tox- 
albumins  and  toxins,  depend  upon  the  closer  resemblance 
such  compounds  bear  to  substances  concerned  in  the 
nutrition  of  the  cells.  At  all  events,  when  an  attempt 
is  made  to  habituate  the  organism  to  the  microorgan- 
ismal products,  greater  success  attends,  and  it  is  found 
that  each  administration  is  followed  by  a  definite  re- 
action which  results  in  a  definite  change  in  the  physio- 
logico-chemical  relationships. 

Suppose,  for  example,  an  experiment  be  performed 
with  the  venom  of  the  cobra.  If  this  poison,  which  is  a 
toxalbumin  and  of  which  a  minute  quantity  is  fatal 
when  injected  beneath  the  skin  or  into  the  circulation, 
is  swallowed  by  a  healthy  warm-blooded  animal,  no- 
harm  is  done,  presumably  because  it  undergoes  diges- 
tion in  the  stomach  and  intestines  and  is  thus  rendered 
harmless.  When  it  is  injected  beneath  the  skin  in  doses 
so  small  as  not  to  produce  death,  the  animal  is  made 
ill,  presents  a  definite  train  of  symptoms,  recovers,  and 
may  then  be  injected  with  a  much  larger  dose.  After 
a  second  illness  or  reaction,  from  which  it  is  permitted 
to  recover,  a  still  larger  quantity  may  be  administered 
with  similar  effects,  and  so  on  until  perhaps  a  thousand 
times  as  much  may  be  given  without  injury  as  would 
have  killed  it  as  a  first  dose. 


376  BIOLOGY:  GENERAL  AND  MEDICAL 

This  kind  of  treatment,  known  as  immunization,  is 
entirely  artificial  and  it  is  doubtful  whether  anything 
like  it  can  occur  in  nature.  It  leads,  however,  to  a 
physiologico-chemical  change  in  the  experiment  animal 
for  its  blood  is  now  found  to  contain  a  new  substance 
by  virtue  of  which  the  poisonous  power  of  the  venom  is 
neutralized  so  that  if  some  of  the  venom  and  some  of 
the  blood  serum  be  mixed  together  and  injected  into  a 
new  animal,  no  effect  is  noticed  if  the  mixture  be  made 
in  proper  proportions.  Not  only  is  this  true  with  regard 
to  one  fatal  dose,  but  for  multiples  of  that  dose,  so  that 
twice  the  fatal  dose,  with  twice  the  necessary  quantity 
of  the  neutralizing  serum,  or  five  times  the  fatal  dose, 
with  five  times  the  quantity  of  the  neutralizing  serum, 
or  perhaps  even  one  hundred  or  one  thousand  fatal 
doses  with  one  hundred  or  one  thousand  times  the 
quantity  of  the  neutralizing  serum  may  be  administered 
without  injury.  Thus  as  the  result  of  the  reactions 
wrought  by  the  venom,  an  antidote  or  neutralizing  sub- 
stance has  been  produced. 

Substances  capable  of  effecting  reactions  attended  by 
such  results  are  known  as  antigens,  the  substances  result- 
ing from  the  reactions  as  anti-bodies. 

Antigens  embrace  a  great  variety  of  substances,  many 
of  which  appear  quite  inert  or  harmless,  as  white  of 
egg,  milk,  peptone  etc.,  yet  all  of  which  are  capable 
under  certain  circumstances  of  bringing  about  systemic 
alterations,  some  of  which  must  be  profound. 

Thus,  the  blood  serum  of  the  horse  when  injected  into 
guinea-pigs  is  harmless.  An  ordinary  adult  guinea-pig 
may  safely  be  injected  with  10  c.c.  or  even  20  c.c.  with- 
out danger  to  life.  Or  guinea-pigs  may  be  given  small 
doses — 1  c.c. — every  few  days  for  an  indefinite  period 
without  harm;  but  if  the  manipulation  be  modified,  most 
unexpected  and  extraordinary  results  may  follow. 
Thus,  suppose  the  guinea-pig  be  injected,  into  the  peri- 
toneal cavity,  with  1/250  c.c.  of  the  horse  serum  and 
then  neglected  for  about  two  weeks.  No  effect  can  be 


INFECTION   AND   IMMUNITY  377 

noted  after  the  injection:  the  animal  appears  well,  con- 
tinues well,  and  shows  no  sign  of  having  experienced  any 
disturbance.  It  has,  however,  undergone  a  profound 
constitutional  change  in  which  the  physiologico-chemical 
balance  has  been  completely  upset,  for  if  it  be  now  given 
a  second  injection  of  only  0.1  to  0.2  c.c.  of  the  same 
serum,  it  within  a  few  moments  becomes  greatly  dis- 
tressed, seems  to  suffer  from  embarrassed  circulation, 
violently  scratches  the  face  and  nose,  gasps  for  breath, 
falls  upon  its  side  more  or  less  convulsed,  and  may  die 
within  an  hour.  This  condition  is  ascribed  to  a  hypcr- 
sensitivity  to  the  horse  serum  effected  by  the  first  or  sensi- 
tizing dose,  and  is  described  as  allergia  or  anaphylaxis. 
Thus  the  single  large  dose  is  not  followed  by  visible  effects; 
frequent  small  doses,  coming  one  after  another  too 
frequently  to  permit  anaphylaxis,  result  in  immunity  to 
the  ill  effects;  but  the  second  dose,  properly  spaced 
after  the  sensitizing  dose  has  effected  its  disturbance, 
is  fatal.  The  nature  of  the  anaphylactic  reaction  is 
not  understood,  but  the  disturbances  resulting  from  it 
are  profound  and  are  accompanied  by  histological  altera- 
tions affecting  many  of  the  tissues  of  the  animal  and 
abundantly  explaining  its  death. 

Reactions  of  this  kind  are  effected  by  many  het- 
erologous  protein  substances,  though  they  are  rarely 
so  profound  or  so  serious  as  in  the  case  chosen  for 
illustration. 

The  quality  of  the  antigen  determines  the  nature 
of  the  antibody  formed  by  the  reaction  following  its 
administration,  and  in  order  that  the  full  force  of  the 
antigen  may  be  effected  it  is  usually  necessary  that  it  be 
admitted  to  the  blood  directly  by  absorption  from  the 
subcutaneous  tissue  or  from  one  of  the  serous  cavities 
rather  than  by  introduction  into  the  alimentary  tract 
where  it  is  apt  to  be  transformed  and  rendered  inert. 

The  reader  must  not  jump  to  the  hasty  and  erroneous 
conclusion  that  any  heterologous  substance  may  serve 
as  an  antigen,  and  produce  antibody  formation.  Only 


378  BIOLOGY:  GENERAL  AND  MEDICAL 

those  substances  are  antigens  that  are  capable  of  effect- 
ing the  essential  physiologico-chemical  reactions  upon 
which  the  antibody  formation  depends.  The  number 
of  antigens  is,  however,  large,  and  they  form  a  highly 
miscellaneous  collection,  embracing  bodies  usually  looked 
upon  as  inert  or  inoffensive,  bodies  highly  poisonous, 
and  even  formed  bodies,  such  as  various  cells — red  blood 
corpuscles,  tissue  comminutions — and  even  the  micro- 
parasites  themselves. 

In  all  of  the  cases  the  antigen  stimulates  the  forma- 
tion of  an  antibody  inimical  to  itself.  If  it  be  a  toxin, 
to  the  formation  of  a  neutralizing  body  or  antitoxin;  if 
it  be  enzyme,  to  the  formation  of  an  antienzyme;  if  a 
cell  or  formed  body,  to  the  formation  of  a  cytotoxin  or 
cell-dissolving  body.  Thus  .the  antibody  is  always 
specific;  i.e.,  reactive  upon  its  antigen  alone. 

The  presence  of  the  respective  antibody  in  its  blood 
confers  increased  resisting  powers  upon  the  animal  in 
whose  blood  it  is,  hence  its  acquired  immunity.  In 
rare  cases  the  immunity  disappears  and  the  animal 
develops  a  mysterious*  hypersensitivity  not  accounted 
for  by  anaphylaxis  as  ordinarily  understood. 

By  long-continued  immunization  of  large  animals  to 
certain  Antigens,  such  as  diphtheria  toxin,  tetanus  toxin, 
venom,  etc.,  we  may  be  able  to  secure  sufficient  quanti- 
ties of  antibodies — antitoxins — to  be  made  practical 
use  of  for  the  treatment  of  the  respective  intoxications — 
diphtheria,  tetanus,  and  snake  bite. 

The  reaction  between  antigen  and  antibody  is  chemical 
in  nature,  but  varies  in  quality.  The  reaction  between 
toxin  and  antitoxin  is  direct  and  immediate;  that  be- 
tween the  antigens  and  the  various  cytotoxins  indirect 
or  intermediate — i.e.,  taking  place  only  in  the  presence 
of  a  third  substance. 

Such  indirect  chemical  actions  are  in  perfect  accord 
with  other  physiological  processes,  and  the  reader  will 
recall  that  pepsin  can  only  act  upon  proteins  in  the 
presence  of  HC1;  trypsin  upon  proteins  only  in  the  pres- 


INFECTION    AND    IMMUNITY  379 

ence  of  enterokynase;  that  rennet  coagulates  casinogen 
only  in  the  presence  of  calcium  salts,  and  the  coagulating 
ferment  of  the  blood  transforms  fibrinogen  only  in  the 
presence  of  a  calcium  salt. 

The  indirect  reactions  are  chiefly  known  in  reference  to 
the  solution  of  formed  bodies — cells,  etc. — and  were  first 
studied  with  reference  to  the  hemolysis  or  the  solution 
of  red  blood  corpuscles  by  immune  hemolytic  serum. 

Such  a  serum  may  be  prepared  by  defibrinating  blood, 
sedimenting  the  corpuscles  with  the  aid  of  a  centrifuge, 
pouring  off  the  plasma,  adding  an  equal  volume  of  phy- 
siological salt  solution,  shaking  up  the  corpuscles  so  as  to 
Wash  them,  recollecting  them  by  means  of  the  centrifuge, 
decanting  the  solution,  replacing  it  by  a  similar  volume 
of  salt  solution,  again  shaking  the  corpuscles  in  the  fluid, 
and  repeating  the  process  once  more.  After  being  thus 
washed,  and  finally  distributed  through  a  small  quantity 
of  salt  solution,  the  suspension  is  injected  into  the  abdom- 
inal cavity  of  a  rabbit.  The  best  treatment  seems  to  be 
to  administer  about  six  doses,  increasing  from  3  to  6  c.c., 
the  injection  being  made  bi-weekly,  always  with  fresh 
material.  In  getting  rid  of  these  corpuscles,  which  act 
as  an  antigen,  an  antibody  known  as  an  immune  body  or 
amboceptor  is  formed.  By  French  writers,  for  reasons 
later  to  be  explained,  it  is  also  known  as  the  fixateur  or 
substance  sensibilisatrice. 

When  washed  red  corpuscles,  prepared  as  has  been 
described,  but  in  a  5  per  cent,  suspension  in  physiolog- 
ical salt  solution,  are  added  to  diluted  blood  serum  from 
an  animal  treated  as  has  been  suggested,  and  therefore 
containing  the  specific  amboceptor,  scarcely  any  change 
will  be  noted,  and  if  the  amboceptor  serum  have  been 
previously  heated  for  an  hour  to  55°  C.,  no  result  at  all 
will  be  effected.  If,  however,  to  the  mixture  that  thus 
appears  to  be  indifferent  there  be  added  a  small  quantity 
of  the  blood-serum  of  a  freshly  killed  guinea-pig,  the 
corpuscles  quickly  hemolyse  and  dissolve.  Upon  inves- 
tigation, the  blood  serum  of  the  normal  guinea-pig  is 


380  BIOLOGY:  GENERAL  AND  MEDICAL 

found  to  contain  the  third  element  required,  which  is 
known  as  the  complement.  This  complementary  sub- 
stance is,  therefore,  something  normal  to  the  blood, 
whose  presence  does  not  depend  upon  any  experimental 
manipulation  and  is  not  capable  of  effecting  any  change 
in  the  blood  corpuscles  by  itself.  It  is,  however,  acti- 
vated by  the  amboceptor.  If  the  process  of  hemolysis 
by  complement  and  amboceptor  is  to  be  thoroughly 
understood,  it  may  be  well  to  visualize  it  in  a  manner 
shown  in  the  following  diagram: 


n  :> 


FIG.  132. — Diagram  illustrating  the  factors  concerned  in  hemolysis,  cytolysis, 
and  bacteriolysis,  d,  The  cell  to  be  dissolved;  c,  the  complement  or  solvent  by 
which  it  is  to  be  dissolved;  a,  the  amboceptor  or  intermediate  body  by  which 
the  two  can  be  brought  together. 

It  is  now  apparent  why  the  amboceptor  is  so  called, 
for  it  is  shown  to  take  hold  of  the  complement  on  one 
hand  and  of  the  corpuscle  on  the  other.  It  will  become 
clear  why  the  complement,  or  solvent,  an  enzymic  sub- 
stance, is  unable  to  accomplish  anything  by  itself,  not 
being  able  to  connect  with  the  corpuscle,  and  why  the 
amboceptor  not  having  solvent  properties  in  itself,  can 
do  nothing,  though  it  may  connect  with  the  corpuscles. 

Great  interest  attaches  to  the  source  of  the  antibodies. 
Undoubtedly  they  are  of  histogenetic  and  of  cellular 
derivation.  It  was  originally  insisted  that  they  were 
derived  from  those  cells  for  which  the  antigen  had  some 
specific  affinity,  but  this  view  has  gradually  given  place 
to  the  opinion  that  many  cells  participate  in  their  forma- 
tion. 

A  theory  of  antibody  formation  known  as  the  lateral 
chain  theory,  suggested  by  Paul  Ehrlich,  has  been  ex- 


INFECTION    AND    IMMUNITY 


381 


tremely  useful  in  enabling  the  student  to  think  clearly 
upon  the  genesis  and  operation  of  the  antibodies,  and 
therefore  has  been  exceedingly  popular  for  a  decade  or 
more. 

The  investigations  began  with  a  study  of  the  chemical 
constitution  of  the  diphtheria  toxin,  and  it  is  important 
to  remember  that  the  experiments  were  made  with  toxic 
antigens  before  other  antigens  and  antibodies  were  well 
known.  Ehrlich  found  that  the  diphtheria  toxin  was 
not  stable,  but  lost  its  toxic  properties  with  the  lapse 
of  time.  This  did  not,  however,  prevent  it  from  com- 
bining with  the  antibody  in  the  usual  proportions,  so 
that  it  seemed  as  though  the  molecules  of  the  toxin  were 
possessed  of  dual  qualities  which  might  be  described  as 
poisoning  and  combining,  respectively.  To  the  poi- 
soning qualities  he  applied  the  term  toxophores,  to  the 
combining  qualities,  haptophores.  In  Ehrlich's  imagina- 
tion a  toxin  molecule  (its  chemical  composition  being 
unknown  and  therefore  impossible  to  represent  by  chem- 
ical symbols)  is  pictured  thus: 


Haptojjhore 
group. 


Toxophore 


TZxcfiiU 
group. 


FIG.  134. 


He  conceives  that  the  toxin'  molecules  attach  them- 
selves to  the  cell  protoplasm  by  virtue  of  receptors  or 
hypothetical  processes  possessing  adaptations  to  such 
molecular  combinations  as  are  utilized  by  the  cells  in 
their  nutrition  and  function,  and  incidentally  to  the 


382 


BIOLOGY:  GENERAL  AND  MEDICAL 


haptophores  of  the  toxins.     This  is  made  clear  by  refer- 
ence to  the  following  ideogram : 


FIG.  135. 

In  cases  in  which  distinct  intoxication  of  the  cell  is 
effected  by  the  toxin  molecule,  not  only  do  the  hap- 
tophorous  groups  attach  themselves  to  the  adapted  h^p- 
tophilic  receptors,  but  the  toxophorous  groups  also  find 
attachment  to  adapted  toxophilic  receptors: 


FIG.  136. — Cell  with  haptophorous  group  attached  to  the  haptophile  and 
toxophore  to  the  toxophile  group,  respectively. 

It  can  be  conjectured  that  when  the  haptophorous 
groups  seize  upon  the  adapted  receptors,  the  cell  proc- 
esses are  embarrassed  through  the  inability  of  the  cell 
to  absorb  its  customary  nutrient  molecular  groups, 
which  are  interfered  with  by  the  presence  of  the  toxin 
molecules,  which,  though  adapted  for  combination  with 
the  receptors,  are  of  no  use  to  the  cell.  To  prevent 
starvation  the  cell  is  supposed  by  Weigert  and  Ehrlich  to 


INFECTION    AND    IMMUNITY  383 

compensate  by  the  regenerative  formation  of  additional 
receptors  to  meet  the  emergency. 

The  period  of  " reaction"  following  the  injection  of 
the  antigen  corresponds  to  the  time  during  which  the 
cells  are  thus  overcoming  the  embarrassment  and  provid- 
ing themselves  with  the  needed  receptors.  As  the  in- 
jections of  the  antigen  are  repeated  and  the  doses  in- 


FIG.  137.  FIG.  138. 

Fios.  137,  138. — The  left  hand  figure  shows  a  cell  embarrassed  by  the  attachment 
of  haptophorous  groups:  the  right  hand  figure  a  cell  compensating  by  regeneration  of 
the  receptors,  some  of  which  can  be  seen  detaching  into  the  surrounding  tissue  juice. 

creased,  the  number  of  new  receptors  to  be  formed 
becomes  greater  and  greater,  and  the  habit  of  regen- 
erating them  so  effectually  established  that  they  form  in 
excess  of  all  requirements,  and  being  superfluous  detach 
from  the  cell  and  occur  free  in  the  lymph  and  blood. 
These  free  receptors  retain  their  haptophilic  affinity  and 
their  haptophorous  adaptation,  so  that  should  adapted 
haptophiles  be  present  they  combine  with  them  in  the 
blood,  before  they  are  able  to  reach  the  cells,  or  being 
present  in  the  drawn  blood  confer  upon  it  the  future 
power  of  combining  with  the  toxin  molecules  rendering 
the  toxin  inert  when  injected,  after  the  combinations 
have  been  effected,  into  some  new  animal. 

The  antitoxic  nature  of  the  immune  serum  is  thus 
referable  to  the  liberated  superfluous  receptors  with 
which  the  blood  serum  of  the  immunized  animal  becomes 


384 


BIOLOGY:  GENERAL  AND  MEDICAL 


more  and  more  thoroughly  charged  as  the  immunization 
process  is  pushed  to  its  maximum  point. 

A  brief  consideration  of  the  subject  will  show  that  the 
natural  immunity  of  any  animal  may  depend  upon 
its  cells  being  without  haptophile  groups,  or  receptors, 
with  the  necessary  adaptations  to  the  toxic  haptophores, 
or  being  without  toxophilous  receptors  by  which  the 
actual  poisonous  combinations  can  be  effected.  It  also 
explains  acquired  immunity  through  regeneration  of  the 
haptophile  groups,  and  the  occurrence  of  the  antitoxic 


FIG.  139.  Fia.  140. 

Fias.  139,  140. — The  cell  on  the  left  hand,  having  regenerated  great  numbers  of 
receptors  for  which  there  is  no  immediate  use,  detaches  them  into  the  tissue  juice 
or  blood;  on  the  right  hand  these  same  detached  receptors  meet  haptophores  in  the 
blood,  with  which  they  combine,  thus  preventing  these  elements  from  reaching  the 
cells.  The  combination  shown  corresponds  with  the  toxin-antitoxin  combination, 
and  may  take  place  as  well  in  in  vitro  as  in  the  body  of  an  animal. 

quality  of  the  blood  of  the  immunized  animals  through 
the  liberation  of  the  superfluous  receptors  into  the  blood. 

The  theory  is  ingeniously  modified  to  explain  those 
different  reactions  that  follow  the  employment  of  an 
antigen  consisting  of  organized  bodies.  To  meet  this 
requirement  it  is  assumed  that  there  is  a  second  order  of 
receptors  possessing  double  affinities,  attracting  on  one 
hand  the  molecules  useful  to  the  cell,  and  on  the  other 
the  enzymic  substances  by  which  they  may  be  utilized: 

Such  receptors,  later  detached  and  circulating  in  the 
blood,  may  be  recognized  as  the  amboceptors  through 
whose  affinity  for  the  complement  on  the  one  hand 
and  for  the  formed  elements  of  the  antigen  on  the 


INFECTION   AND   IMMUNITY 


385 


other  the  interaction  resulting  in  solution  or  cytolysis 
is  brought  about. 

It  seems  unnecessary  to  pursue  the  ramifications  of 
the  theory.  It  is  a  very  pregnant  one  and  has  been  of 
inestimable  value  in  enabling  physiological  chemists 
to  grasp  the  facts  where,  the  true  chemistry  of  the  re- 
actions being  unknown,  it  might  not  have  been  possible 
to  follow  them  along  conventional  lines. 

But  it  will  be  remembered  that  our  study  of  the 
problems  of  immunity  began  with  the  resistance  of 
certain  organisms  to  microparasites 
by  which  others  are  successfully 
invaded  and  destroyed,  and  though 
these  microorganisms  were  men- 
tioned as  the  chief  factors  for  con- 
sideration, a  digression  was  made 
in  order  that  the  facts  appertain- 
ing to  immunity  from  intoxication 
might  be  thoroughly  in  mind,  for 
it  is  almost  axiomatic  that  ability 
to  resist  infection  implies  ability  to 
endure  the  toxic  products  of  the 
microparasite. 

We  may  therefore  be  justified  in 
supposing  that  when  an  immune 
animal  is  found  to  destroy  micro- 
parasites  in  its  body,  it  must  be  in- 
different to  their  toxic  products. 
Its  cells  experiencing  no  injury  are 
and  destroy  the  invading  microorganisms. 

The  destruction  of  the  microparasites  in  immune  ani- 
mals is  effected  in  two  ways:  1,  by  the  activity  of  the 
phagocytic  cells  which  devour  them;  2,  by  solution  in 
the  body  juices.  Both  methods  are  usually  observed, 
though  the  former  is  most  frequent  in  naturally  immune 
animals,  the  latter  in  animals  with  acquired  immunity. 

Though  perhaps  not  first  observed  by  him,  phagocy- 
tosis was  first  suggested  by  EH6  Metchnikoff  as  the  chief 


FIG.  141. — Surface  of  a 
cell  with  a  receptor  of  the 
second  order  fitted  to  com- 
bine on  one  hand  with  a, 
an  albuminous  molecule, 
and  b,  an  enzymic  mole- 
cule. (Ehrlich  and  Mar- 
shall.) 

free  to   attack 


386  BIOLOGY:  GENERAL  AND  MEDICAL 

defense  of  the  body  against  infection.  His  original  ob- 
servations were  made  upon  certain  water  fleas  invaded 
by  a  fungus.  In  those  cases  in  which  the  microparasite 
was  overcome,  the  phagocytic  cells  of  the  little  animal 
were  so  active  that  he  attributed  its  salvation  to  them. 

Through  years  of  patient  investigation  Metchnikoff 
and  his  followers  have  brought  together  a  tremendous 
array  of  facts  in  support  of  the  theory  of  phagocytosis, 
and  have  entrenched  themselves  behind  such  conclusive 
evidence  as  to  be  almost  unassailable. 

wm T          : n 


FIG.  142. — Phagocytosis;  the  omentuin,  immediately  after  injection  of 
typhoid  bacilli  into  a  rabbit.  Meahwork  showing  a  macrophage,  intermediate 
forms,  and  a  trailer,  all  containing  intact  bacilli.  (Buxton  and  Torrey.) 

The  theory  naturally  underwent  many  modifications 
as  necessity  for  them  arose,  but  throughout  the  years  of 
enthusiastic  appreciation  that  followed  the  development 
of  the  lateral  chain  theory,  Metchnikoff  has  remained 
unshaken  in  the  belief  that  the  reactions  of  immunity 
are  simple  and  has  continued  to  maintain  that  they  are 
all  finally  referable  to  the  phagocytes. 

It  becomes  imperative,  therefore,  that  the  theory  o/ 
phagocytosis  be  carefully  examined  in  order  that  its 
merits  as  explanations  of  the  phenomena  of  immunity 
can  be  estimated.  As  originally  conceived,  phagocytosis 


INFECTION   AND   IMMUNITY  387 

implied  that  the  cells  of  the  host,  especially  the  white 
corpuscles  of  the  blood,  took  the  microparasites  into  their 
substance  and  destroyed  them  just  as  an  amoeba  takes 
up  many  small  objects,  digests,  and  dissolves  them. 

An  investigation  of  the  leucocytes  of  immune  animals 
shows  that  though  there  are  notable  exceptions,  the 
blood  corpuscles  do  behave  in  this  manner.  It  is  also 
found,  though  again  there  are  exceptions,  that  the  cor- 
puscles of  susceptible  animals  usually  neglect  the  micro- 
parasites,  which  are  therefore  free  to  multiply;  but  that 
when  such  naturally  susceptible  animals  are  by  any 
means  made  immune,  their  corpuscles  change  and 
begin  to  take  up  the  microparasites. 

The  conditions  thus  corresponded  fairly  well  with 
the  requirements  of  the  theory  until  certain  additional 
facts  were  discovered. 

Thus  it  was  found  by  Nuttall  and  Buchner  that  bacteria 
are  commonly  killed  when  placed  in  the  blood  serum 
of  an  animal.  This  led  to  a  new  idea — that  the  bacteria 
were  killed  by  some  substance  in  the  body  juices  and 
only  taken  up  by  the  phagocytes  after  death  had  made 
them  harmless.  Many  interesting  and  some  paradoxical 
observations  were  made.  Thus  the  bacillus  of  anthrax 
when  put  into  the  rabbit's  body  rapidly  multiplies, 
distributes  itself  throughout  the  blood,  reaches  all  the 
organs,  and  kills  the  rabbit.  If,  however,  the  anthrax 
bacilli  are  placed  in  some  of  the  rabbit's  blood  drawn 
into  a  test-tube,  they  meet  with  speedy  destruction. 
Why  should  this  paradox  occur?  Why  should  their 
introduction  into  the  rabbit's  blood  in  the  rabbit's  body 
be  followed  by  the  death  of  the  rabbit,  but  their  intro- 
duction into  the  rabbit's  blood  in  a  test-tube  be  followed 
by  their  own  death?  This  was  a  much  debated  point. 
Buchner  and  his  followers  named  the  bacteria-destroy- 
ing substance  of  the  blood  alexine.  For  a  while  it  seemed 
as  though  the  doctrine  of  phagocytosis  had  received 
its  death-blow,  but  Metchnikoff  replied  that  the  destruc- 
tion of  the  bacteria  by  the  phagocytes  depended  upon 


388  BIOLOGY:  GENERAL  AND  MEDICAL 

enzymic  substances,  and  that  if  the  phagocytes  were 
destroyed  the  enzymes  remained  in  solution  in  the  body 
juices.  The  alexine  of  Buchner  was,  in  his  opinion, 
microcytase,  an  enzyme  resulting  from  phagolysis,  or 
dissolution  of  the  phagocytes. 

By  an  ingenious  manipulation,  he  contrived  to  place 
bacteria,  inclosed  in  small  collodion  bags,  in  the  body 
cavities  of  animals.  These  microorganisms,  though 
exposed  to  the  effects  of  the  body  juices,  were  defended 
from  the  phagocytes  and  multiplied  abundantly,  though 
if  the  bag  ruptured  they  were  destroyed  by  the  cells. 

In  the  meantime,  the  toxins  and  antitoxins  had  been 
discovered,  and  it  became  necessary  to  account  for  im- 
munity against  intoxication  and  for  antitoxin  formation. 
The  theory  was,  however,  capable  of  application  to  the 
new  problems,  for,  it  was  argued,  not  only  may  the 
phagocytic  cells  take  up  formed  objects,  but  they  may 
also  absorb  fluids,  and  by  virtue  of  their  enzymes,  digest 
and  destroy  the  toxins  they  contain.  Further,  the 
enzymes  liberated  from  destroyed  phagocytes  may  be 
capable  of  acting  similarly  upon  free  toxin  in  the  blood. 

Each  toxin  injection  administered  to  an  animal  was 
supposed  to  destroy  innumerable  phagocytes,  with  whose 
enzymic  contents  the  blood  became  replete,  hence  its 
antitoxic  character. 

It  was  sometimes  difficult  to  substantiate  the  views 
of  the  theorist,  but  time  usually  brought  forth  new  and 
surprising  evidences  in  his  favor. 

Two  kinds  of  phagocytes  were  next  found  to  be  im- 
portant, the  leucocytes  or  microphages,  possessing  an 
enzyme  called  microcytase,  and  the  tissue  cells,  notably 
the  endothelial  cells  or  macrophages,  possessing  an  enzyme 
called  macrocytase. 

The  bacteria  and  minute  entities  are  taken  up  by  tht 
microphages,  larger  objects,  like  heterologous  blood 
corpuscles,  suspended  tissue  fragments,  etc.,  by  the 
macrophages.  The  action  of  the  two  enzymes  is  slightly 
different. 


INFECTION   AND   IMMUNITY  389 

When  the  agglutination  of  bacteria  was  discovered, 
and  observers  everywhere  were  trying  to  account  for  the 
peculiar  phenomenon,  Metchnikoff  attributed  it  to  the 
action  of  ono  of  these  enzymes  and  looked  upon  it  as  a 
preparation  of  phagocytosis. 

But  the  theory  expanded  most  beautifully  when  it 
became  necessary  to  account  for  the  phenomena  of 
cytolysis.  Ehrlich's  explains  it  as  resulting  from  the 
combination  of  complement,  amboceptor,  and  cell. 
Metchnikoff  regards  it  as  the  result  of  the  successive 
action  of  the  two  enzymes.  One,  probably  the  macro- 
cystase,  prepares  the  cell  by  fixing  or  sensitizing  it,  the 
other,  the  microcytase,  then  dissolves  it.  For  this  rea- 
son Metchnikoff  never  uses  the  term  amboceptor,  but 
speaks  of  that  factor  in  cytolysis  as  the  fixateur  or  sub- 
stance sensibilisatrice. 

When  Wright  discovered  certain  substances  in  the 
blood  which  he  called  opsonins,  and  which  he  believes 
prepare  the  bacteria  for  phagocytosis,  it  seemed  to 
Metchnikoff  but  a  new  application  of  the  fixateur. 

There  are,  therefore,  two  means  by  which  the  infect- 
ing microparasites  may  be  destroyed  in  naturally 
immune  animals — the  phagocytic  cells  and  the  germi- 
cidal  body  humors — and  it  makes  comparatively  little 
difference  by  what  theories  we  account  for  their 
action. 

We  must  next  endeavor  to  find  out  whether  the  same 
conditions  obtain  in  acquired  immunity,  but  before  at- 
tempting this  it  may  be  well  to  pause  to  inquire  how 
immunity  against  infection  may  be  acquired. 

It  has  already  been  pointed  out  that  there  are  many 
diseases  from  which  one  usually  suffers  but  once. 
Though  a  few  notable  exceptions  occur,  it  is  well  known 
that  to  have  had  chicken-pox,  measles,  scarlatina,  mumps, 
whooping-cough,  yellow  fever,  typhoid  fever,  and  small- 
pox is  to  be  immune  against  future  attacks.  The  ex- 
ceptions are  of  interest  because  they  coincide  with  the 
results  of  experimental  investigation. 


390  BIOLOGY:  GENERAL  AND  MEDICAL 

In  reference  to  all  these  diseases  we  can  make  the 
following  statements: 

There  are  a  few  persons  who,  having  been 
exposed  to  one  or  the  other  of  the  infectious 
agents,  escape  illness. 

There  are  a  few  persons  who,  having  been  ex- 
posed once  or  even  several  times  without 
resulting  illness,  succumb  upon  an  additional 
exposure. 

There  are  a  few  persons  who,  having  been 
infected  and  suffered  the  illness,  take  it  again 
after  the  lapse  of  a  varying  length  of  time. 
There  are  a  great  number  who,  having  once' 
suffered  from  one  of  these  diseases,  never  take 
it  again. 

These  general  statements  show  that  there  are  differ- 
ences in  the  behavior  of  different  individuals  toward 
the  infectious  diseases.  They  also  show  that  acquired 
immunity  is  less  permanent  and  less  uniform  than  natural 
immunity.  Some  persons  acquire  little  immunity 
through  infection,  some  soon  lose  the  immunity,  some 
retain  it  many  years  and  then  lose  it,  some  never  lose  it. 

Experience  also  leads  us  to  believe  that  the  per- 
manence of  immunity  bears  some  reference  to  the 
severity  of  the  disease;  to  have  a  disease  badly  may, 
but  does  not  necessarily,  guarantee  a  more  thorough 
and  more  prolonged  immunity  than  to  have  it  very 
lightly. 

It  may  be  imagined  that  so  soon  as  it  became  clear 
that  to  have  a  contagious  disease  afforded  immunity 
from  future  attacks,  sagacious  individuals  set  about 
devising  means  by  which  practical  advantage  might  be 
made  of  the  information.  Modern  methods  of  experi- 
ment were,  however,  unknown,  and  the  only  possible 
application  during  many  centuries  was  the  occasional 
exposure  of  healthy  persons  to  mild  cases  of  the  infec- 
tious diseases  in  the  hope  that  they  might  pass  through 


INFECTION   AND   IMMUNITY  391 

a  mild  attack  of  the  disease  at  a  convenient  time.  The 
method  could  not,  in  the  very  nature  of  things,  attain 
to  any  degree  of  popularity,  though  it  is  still  practised 
to  some  extent,  and  children  are  sometimes  during  the 
vacation  season  thus  exposed  to  chicken-pox  and 
measles  in  order  that  they  may  lose  no  time  at  school. 

It  has  been  for  centuries  the  practice  of  the  Chinese  to 
induce  small-pox  by  thrusting  scabs  from  the  pocks 
into  the  noses  of  healthy  persons  or  to  tie  them  upon 
their  persons  to  produce  a  mild  attack  of  the  disease. 
The  method  is  crude  and  filthy  and  likely  to  result 
disastrously.  The  Turks  invented  an  improved  method, 
which  when  brought  to  western  Europe  was  known  as 
"  inoculation "  and  was  practised  with  good  enough 
results  to  be  continued  until  something  better  was 
discovered. 

In  her  letter  to  Mrs.  S.  C. —  — ,  dated  Adrianople, 
April  10,  O.  S.  1717,  Lady  Mary  Wortley  Montague 
writes  upon  this  subject  as  follows:  "The  small-pox, 
so  fatal  and  so  general  among  us,  is  here  entirely  harm- 
less by  the  invention  of  ingrafting,  which  is  the  term 
they  give  it.  There  is  a  set  of  old  women  who  make  it 
their  business  to  perform  the  operation  every  autumn, 
in  the  month  of  September,  when  the  great  heat  is  abated. 
People  send  to  one  another  to  learn  if  any  of  their  family 
has  a  mind  to  have  the  small-pox;  they  make  parties 
for  the  purpose,  and  when  they  are  met  (commonly 
fifteen  or  sixteen  together),  the  old  woman  comes  with  a 
nut-shell  full  of  the  matter  of  the  best  sort  of  small-pox, 
and  asks  what  veins  you  will  have  opened.  She  imme- 
diately rips  open  that  you  offer  her  with  a  large  needle 
(which  gives  you  no  more  pain  than  a  common  scratch) 
and  puts  into  the  vein  as  much  venom  as  can  lie  upon 
the  head  of  her  needle,  and  after  binds  up  the  little 
wound  with  a  hollow  bit  of  shell;  and  in  this  manner 
opens  four  or  five  veins.  The  Grecians  have  commonly 
the  superstition  of  opening  one  in  the  middle  of  the 
forehead,  one  in  each  arm,  and  on  the  breast,  to  mark 


392  BIOLOGY:  GENERAL  AND  MEDICAL 

the  sign  of  the  cross;  but  this  has  a  very  ill  effect,  all 
of  these  wounds  leaving  little  scars,  and  is  not  done  by 
those  that  are  not  superstitious,  who  choose  to  have  them 
in  the  legs  or  that  part  of  the  arm  that  is  concealed. 
The  children  or  young  patients  play  together  all  the 
rest  of  the  day  and  are  in  perfect  health  to  the  eighth. 
Then  the  fever  begins  to  seize  them,  and  they  keep  their 
beds  two  days,  very  seldom  three.  They  have  very 
rarely  above  twenty  or  thirty  in  their  faces,  which  never 
mark;  and  in  eight  days'  time  they  are  as  well  as  before 
their  illness.  Where  they  were  wounded  there  remain 
running  sores  during  the  distemper,  which,  I  don't  doubt, 
is  a  great  relief  to  it.  Every  year  thousands  undergo 
this  operation;  and  the  French  Ambassador  says  pleas- 
antly that  they  take  the  small-pox  here  by  way  of 
diversion  as  they  take  the  waters  in  other  countries. 
There  is  no  example  of  anyone  that  has  died  in  it;  and 
you  may  believe  I  am  very  well  satisfied  of  the  safety 
of  this  experiment,  since  I  intend  to  try  it  on  my  dear 
little  son." 

The  next  experimental  application  of  the  principle  of 
preventing  an  infectious  disease  by  giving  an  attack  of  a 
disease  was  made  by  Edward  Jenner,  an  English  physi- 
cians and  naturalist,  once  a  pupil  of  John  Hunter,  in 
whose  family  he  lived.  For  some  time  Jenner  had  been 
engaged  in  the  study  of  small-pox,  cow-pox,  and  swine- 
pox,  and  the  development  of  the  latter  two  diseases  when 
communicated  to  man.  He  at  first  made  the  mistake 
of  believing  cow-pox  to  be  caused  by  the  contagion  of  a 
peculiar  hoof  disease  of  horses  known  as  "  grease."  He 
first  suggested  that  an  attack  of  cow-pox  would  prevent 
small-pox,  and  that  it  might  be  experimentally  em- 
ployed for  that  purpose,  in  a  conversation  with  William 
Hunter  as  early  as  1770.  A  German  schoolmaster, 
Nicholas  Plett,  had  already  held  the  same  idea  and  had 
made  certain  proofs,  but  the  matter  had  gone  no  further. 
Jenner  inoculated  his  own  son  with  swine-pox  and 
later  found  him  immune  against  small-pox.  Fearing 


INFECTION    AND   IMMUNITY  393 

failure  and  being  timid  by  nature,  it  was  not  until  May 
14,  1796,  that  Jenner  gave  a  public  demonstration.  A 
boy  was  inoculated  with  cow-pox,  and  having  passed 
through  the  disease,  was  found  upon  inoculation  to  be 
immune  against  small-pox.  In  1798,  Jenner  wrote 
a  lengthy  paper  upon  "vaccination,"  detailing  the  whole 
matter  and  stating  his  belief  and  his  proofs.  For  a  long 
time  the  medical  profession  as  well  as  the  laity  were 
incredulous,  but  experiments  were  made  by  one  after 
another  of  the  prominent  physicians  and  the  method 
slowly  spread  until  its  advantages  became  so  apparent 
that  it  became  adopted  in  one  after  another  of  the 
European  countries  and  finally  in  America,  ultimately 
being  made  more  or  less  compulsory  in  all  countries 
with  such  striking  success  that  small-pox,  once  the  most 
frequent  and  most  terrible  of  maladies,  has  become  a 
comparatively  rare  disease  in  most  civilized  countries. 

From  the  fact  that  the  disappearance  of  small-pox 
has  been  coincidental  with  the  disappearance  of  cow-pox, 
swine-pox,  etc.,  and  that  the"  inoculated  viruses  of 
these  diseases  afforded  protection  against  small-pox, 
there  can  be  little  doubt  that  these  affections  had  a 
common  ancestry,  and  that  the  mild  character  of 
vaccinia,  or  cow-pox,  in  man  is  due  to  some  change 
suffered  by  the  specific  microparasites — a  diminution  in 
their  virulence — resulting  from  their  exposure  to  the 
defensive  juices,  etc.,  of  the  cow. 

No  further  progress  was  made  in  the  field  of  experi- 
mentally induced  immunity  from  the  invention  of 
vaccination  by  Jenner  until  the  time  of  Louis  Pasteur, 
nearly  one  hundred  years. 

With  Pasteur,  however,  came  a  new  epoch,  that  of 
the  discovery  of  the  microparasites  of  disease,  and  shortly 
after,  through  the  work  of  Koch  and  Pasteur,  the  means 
of  artificially  cultivating  and  accurately  observing  them, 
and  in  consequence  a  great  expansion  in  the  knowledge 
of  infectious  diseases  and  in  the  means  of  preventing 
them. 


394  BIOLOGY:  GENERAL  AND  MEDICAL 

From  chemistry  Pasteur  was  led  into  a  study  of  fer- 
mentation and  putrefaction  which  he  discovered  to  be 
due  to  microorganismal  life,  the  source  of  which  he 
quickly  traced  to  the  spores  or  seeds  of  minute  plants 
abounding  in  the  atmosphere.  The  source  of  fermenta- 
tion being  thus  traceable  to  living  entities  in  the  air, 
he  conjectured  that  the  source  of  fermentation  in  wounds 
might  be  the  same,  and  an  investigation  of  the  discharges 
from  fetid  wounds  showed  them  to  be  teeming  with 
microorganismal  life  capable  of  infecting  the  small 
animals  used  for  inoculation  experiments.  Convinced 
that  these  microbes  were  the  cause  of  the  disturbances, 
the  investigation  was  pursued,  and  for  various  maladies 
different  microbes  were  found.  The  first  investigations 
bearing  directly  upon  the  subject  of  immunity  were 
made  with  the  bacillus  of  chicken  cholera  and  came 
about  in  a  peculiar  manner.  "  A  chance  such  as  happens 
to  those  who  have  the  genius  of  observation  was  now 
about  to  mark  an  immense  step  in  advance  and  prepare 
the  way  for  a  great  discovery.  As  long  as  the  culture 
flasks  of  the  chicken-cholera  microbes  had  been  sown 
without  interruption,  at  twenty-four  hours'  interval, 
the  virulence  had  remained  the  same;  but  when  some 
hens  were  inoculated  with  an  old  culture,  put  away 
and  forgotten  a  few  weeks  before,  they  were  seen,  with 
surprise,  to  become  ill  and  then  to  recover.  These 
unexpectedly  refractory  hens  were  then  inoculated  with 
some  new  culture,  but  the  phenomenon  of  resistance 
had  occurred.  What  had  happened?  What  could 
have  attenuated  the  activity  of  the  microbe?  Re- 
searches proved  that  oxygen  was  the  cause,  and,  by 
putting  between  the  cultures  variable  intervals  of  days, 
of  one,  two,  or  three  months,  variations  of  mortality 
were  obtained,  eight  hens  dying  out  of  ten,  then  five, 
then  only  one  out  of  ten,  and  at  last,  when,  as  in  the  first 
case,  the  culture  had  had  time  to  get  stale,  no  hens 
died  at  all,  though  the  microbe  could  still  be  cultivated." 

"Finally,"  said  Pasteur,  eagerly  explaining  this  phe- 


INFECTION   AND   IMMUNITY  395 

nomenon,  "  if  you  take  each  of  these  attenuated  cultures 
as  a  starting-point  for  successive  and  uninterrupted 
cultures,  all  this  series  of  cultures  will  reproduce  the 
attenuated  virulence  of  that  which  served  as  a  starting 
point;  in  this  same  way  non-virulence  will  produce 
non- virulence." 

"  And,  while  hens  who  had  never  had  chicken  cholera 
perished  when  exposed  to  the  deadly  virus,  those  who  had 
undergone  attenuated  inoculations  and  who  afterward 
received  more  than  their  share  of  the  deadly  virus,  were 
affected  some  with  the  disease  in  a  benign  form,  a  pass- 
ing indisposition,  sometimes,  even,  they  remained  per- 
fectly well;  they  had  acquired  immunity.  Was  not 
this  fact  worthy  of  being  placed  by  the  side  of  that 
great  fact  of  vaccine  over  which  Pasteur  had  so  often 
pondered  and  meditated?" 

Practical  application  for  the  prevention  of  chicken 
cholera  was  soon  made  of  this  observation,  and  it  led  to  the 
next  great  achievement  in  the  way  of  inducing  immunity. 

The  bacillus  of  anthrax  (splenic  fever  of  cattle)  had 
been  discovered  by  Davaine,  and  Pasteur's  great  ambi- 
tion was  to  prepare  some  vaccine  by  which  its  ravages 
might  be  stayed.  The  problem  could  not,  however, 
be  so  easily  solved,  for  the  spores  of  the  bacillus  prevented 
its  attenuation  and  preserved  their  original  virulence 
after  having  been  kept  dry  for  ten  years.  Clearly  some 
other  means  of  attenuation  must  be  devised.  Eventu- 
ally, after  having  tried  many  means  of  effecting  the 
attenuation  necessary  for  the  vaccine,  he  found  that 
when  the  microbes  were  cultivated  at  an  elevated  tem- 
perature— 42°  to  43°  C. — they  did  not  develop  spores, 
and  that  when  cultures  so  modified  were  subsequently 
cultivated  at  30°  C.,  they  retained  this  peculiarity  as 
well  as  diminished  virulence. 

The  result  of  this  observation  was  ability  to  produce 
cultures  of  varying  degrees  of  virulence,  which  like  those 
of  the  chicken-cholera  microbes,  bred  true  to  their 
acquired  virulence. 


396  BIOLOGY:  GENERAL  AND  MEDICAL 

When  the  statement  was  made  by  Pasteur  that  by  the 
use  of  vaccines  consisting  of  properly  selected  cultures  of 
attenuated  anthrax  bacilli,  he  would  be  able  to  prevent 
the  occurrence  of  anthrax,  a  storm  of  opposition  and 
ridicule  was  aroused  but  quickly  quelled  for  on  May  5, 
1882,  at  the  farm  of  Pouiily  le  Fort  near  Melun,  France, 
he  gave  a  large  public  exhibition  and  vaccinated  twenty- 
five  sheep,  five  cows,  and  an  ox  with  the  first  virus, 
before  a  large  gathering  of  agriculturists,  physicians, 
and  veterinarians.  On  May  17,  the  second  inoculation 
\vith  a  more  virulent  virus  was  made.  On  May  31, 
the  final  test  was  made  ,  and  all  of  the  animals  were  inocu- 
lated with  a  triple  dose  of  virulent  virus.  On  June  2 
all  of  the  many  control  animals  were  dead,  but  the 
vaccinated  animals  were  all  well  and  remained  so. 

This  was  the  inception  of  a  method  that  has  saved 
millions  of  dollars  to  the  farmers,  by  enabling  them 
to  protect  their  stock  whenever  anthrax  makes  its 
appearance. 

Almost  at  the  same  time  Arloing,  Cornevin  and  Tho- 
mas, and  Kitt  applied  a  similar  method  for  protecting 
animals  from  quarter-evil.  The  vaccine,  however,  was 
not  made  with  pure  cultures  of  the  microbe,  but  by 
drying  and  heating  the  muscular  tissue  of  an  animal 
inoculated  with  it.  The  muscle  contained  innumerable 
bacilli,  which  attenuated  when  the  dry  muscle  was 
heated  for  a  time.  The  dry  muscle  thus  treated  was 
ground  to  a  powder,  suspended  in  some  indifferent 
fluid,  and  injected  with  a  hypodermic  syringe  into  the 
animal  to  be  protected.  An  injection  of  this  kind  was 
found  to  be  sufficient  to  afford  immunity.  This  method 
is  also  now  in  general  use  and  has  been  of  great  economic 
value  to  agriculturalists. 

Pasteur  next  extended  his  immunological  researches 
to  human  pathology  and  devoted  himself  to  rabies,  or 
hydrophobia,  a  terrible  disease,  invariably  fatal,  caused 
by  the  bites  of  rabid  animals.  He  found  that  the 
virus,  though  present  in  the  saliva  and  transmitted  by  it, 


INFECTION   AND   IMMUNITY  397 

was  so  hopelessly  mixed  with  other  pathogenic  micro- 
organisms of  the  saliva  as  to  make  it  impossible  to  use 
it  as  the  basis  of  exact  experiments.  Looking  for  the 
microbe  of  rabies,  he  found  it  present  in  greatest 
intensity  in  the  nervous  system,  and  found  that  by 
rubbing  up  the  nervous  tissue  from  the  brain  or  spinal 
cord  with  physiological  salt  solution,  it  was  possible  to 
secure  the  virus  in  a  form  free  from  admixture  with  other 
microbes  and  convenient  for  experimental  investigation. 

Unfortunately,  though  there  was  every  evidence  that 
the  disease  was  infectious  and  therefore  microbic,  he 
found  it  impossible  either  to  demonstrate  the  specific 
microbe  by  the  microscope  or  to  make  it  grow  in  artifi- 
cial culture.  In  regard  to  this  it  may  be  well  to  remark 
that  we  are  not  yet  able  to  cultivate  this  microbe,  though 
there  seems  to  be  little  doubt  about  it  being  a  protozoan 
parasite  discovered  by  an  Italian  named  Negri. 

Disregarding  his  inability  to  demonstrate  the  microbe 
and  finding  that  he  was  perfectly  able  to  reproduce  the 
disease  experimentally,  by  using  the  emulsion  of  the 
nervous  tissues,  Pasteur  set  about  finding  methods  of 
attenuating  the  virus.  The  results  were  most  interest- 
ing. The  nervous  tissues  of  dogs  and  other  animals  with 
the  disease  were  found  to  yield  viruses  of  varying  degrees 
of  virulence,  "street  virus";  but  after  such  a  virus  was 
manipulated  in  the  laboratory  by  passage  through  a 
series  of  rabbits,  it  acquired  a  uniform  degree  of  virulence 
and  became  known  as  a  "fixed  virus."  Such  a  fixed 
virus,  contained  in  the  spinal  cord  of  a  rabbit,  was 
further  found  to  be  susceptible  of  attenuation  by  drying, 
the  degree  of  attenuation  being  proportionate  to  the 
length  of  the  period  of  drying.  It  was  not  difficult  to 
arrive  at  any  degree  of  attenuation  until  virulence  was 
eventually  entirely  lost.  By  working  with  the  attenu- 
ated viruses,  and  administering  a  succession  of  doses 
with  increasing  degrees  of  virulence,  animals  could  be 
immunized  against  the  virulent  "street  virus." 

As,  of  course,  no  one  would  desire  to  be  immunized 


398  BIOLOGY:  GENERAL  AND  MEDICAL 

against  the  disease  who  was  in  no  exceptional  danger  of 
getting  it,  no  use  could  be  made  of  his  method  unless  it 
could  be  applied  to  those  in  imminent  danger — i.e.,  who 
had  already  been  bitten  by  mad  dogs.  A  peculiarity  of 
the  disease  is  its  long  incubation  period,  which  varies 
from  one  to  several  months.  The  thought  occurred  to 
Pasteur  that  it  might  be  possible  to  effect  the  immuni- 
zation during  the  incubation  period.  This  method  was 
tried  with  great  hesitation  upon  a  lad  terribly  bitten  by  a 
mad  dog.  "  The  child,  going  to  school  by  a  little  byroad, 
had  been  attacked  by  a  furious  dog  and  thrown  to  the 
ground.  Too  small  to  defend  himself,  he  had  only  thought 
of  covering  his  face  with  his  hands.  A  bricklayer,  see- 
ing the  scene  from  a  distance,  arrived  and  succeeded  in 
beating  off  the  dog  with  an  iron  bar;  he  picked  up  the 
boy  covered  with  blood  and  saliva.  The  dog  went  back 
to  his  master,  Theodore  Vone,  a  grocer  at  Meissengatt, 
whom  he  bit  on  the  arm.  Vone  seized  a  gun  and  shot 
the  animal,  whose  stomach  was  found  to  be  full  of  hay, 
straw,  pieces  of  wood,  etc."  This  little  boy,  Joseph 
Meister,  was  the  first  to  receive  the  treatment,  though  it 
was  undertaken  with  great  reluctance  by  Pasteur  and 
only  upon  the  advice  of  Vulpian  and  Grancher.  The 
lad  escaped  hydrophobia  and  experienced  no  ill  from 
the  treatment.  Other  opportunities  for  testing  the 
method  were  soon  forthcoming,  and  it  was  soon  evident 
that  a  new  triumph  had  been  achieved,  for  in  all  those 
cases  that  came  to  hand  sufficiently  early,  the  disease 
was  prevented  and  in  no  case  was  harm  done.  The 
treatment  is  extremely  simple.  It  consists  in  daily 
injections  of  qmulsions  of  spinal  cord  in  physiological 
salt  solution,  the  cords  being  taken  from  rabbits  inocu- 
lated with  the  "fixed  virus"  and  dried  over  calcium 
chloride  in  sterile  bottles.  The  first  cord  should  have 
dried  about  fourteen  days,  the  next  thirteen  days,  the 
next  twelve,  and  so  on  with  modifications  such  as  the 
experience  of  the  operator  or  the  necessity  of  the  case 
may  make  desirable.  The  success  of  the  method  has 


INFECTION   AND   IMMUNITY  399 

been  so  gratifying  that  "Pasteur  institutes"  for  its  appli- 
cation have  been  founded  by  governments  or  by  large  cities 
in  nearly  all  parts  of  the  world. 

Another  important  application  of  this  method  of 
preventing  disease  by  the  use  of  modified  cultures  of  the 
specific  microorganisms  of  the  disease  has  been  made 
with  great  success  by  a  Russian  bacteriologist  named 
Haffkine,  for  the  prevention  of  cholera.  In  this  case, 
the  specific  organism,  the  "comma  bacillus,"  a  spiral 
organism,  having  long  ago  been  discovered  by  Koch, 
and  being  easily  cultivable,  the  method  of  operating 
was  more  simple  and  more  closely  resembled  the  vaccina- 
tions against  chicken  cholera  and  anthrax. 

The  success  of  Haffkine  probably  stimulated  A.  E. 
Wright  to  attempt  very  much  the  same  method  in  the 
prophylaxis  of  typhoid  fever.  These  two  methods  both 
depend  upon  the  employment  of  attenuated  or  killed 
cultures  for  the  production  of  sufficient  active  immunity 
to  enable  the  recipient  to  resist  ordinary  infection. 

The  same  thing  was  later  tried  by  Haffkine  for  the 
prevention  of  plague,  and  in  all  three  of  these  diseases, 
cholera,  typhoid  fever,  and  plague,  trials  made  upon 
large  numbers  of  soldiers  have  shown  most  gratifying 
results. 

A  still  further  utilization  of  the  principle  has  been 
made  by  Wright  in  the  "vaccine  treatment"  of  many 
of  the  infectious  diseases.  The  fundamental  idea  being 
that  when  the  disease  is  of  prolonged  duration,  or  of 
circumscribed  invasiveness,  the  vaccination  of  the 
patient  with  killed  or  attenuated  cultures  of  the  specific 
organism  brings  about  a  sudden  and  acute  reaction, 
followed  by  an  increase  in  the  general  resisting  power 
through  improvement  of  the  bacteria-destroying  mechan- 
ism by  which  the  bacteria  may  be  overcome.  Excellent 
results  are  claimed  for  this  method  in  the  treatment  of 
suppurating  acne,  furunculosis,  certain  localized  forms 
of  tuberculosis,  various  chronic  suppurating  sinuses,  etc. 

The   nature   of  the  resisting  power  thus  induced  is 


400  BIOLOGY:  GENERAL  AND  MEDICAL 

found  to  depend  upon  a  combination  of  those  factors 
engaged  in  the  defense  of  the  body.  That  is,  there  is  a 
trace  of  antitoxic  power  in  the  blood  in  those  cases  in 
which  toxic  substances  were  embraced  in  the  antigen; 
amboceptors  are  present  when  the  antigen  contains  the 
essential  microbes  of  the  affection,  but,  above  all,  in  all 
cases  of  active  immunity  against  infection  the  phagocytic 
power  of  the  leucocytes  is  greatly  increased  so  that  the 
cells,  originally  inactive  or  feebly  active,  become  very 
active  and  hungry  for  the  microorganisms  which  they 
greedily  devour  and  destroy. 

REFERENCES, 

R.    KRAUS    and    C.    LEVADITI:      "Handbuch    der  Technik    und 

Methodik  der  Immunitatsforschung,"  Jena,  1909. 
ELIE  METCHNIKOFF:      "  L/Immunite"    dans    les    maladies    infec- 

tieuses,"  Paris,  1901. 

LUDWIG  ASCHOFF:  "Ehrlichs  Seitenkettentheorie,"  Jena,  1905. 
JOSEPH  MCFARLAND:  "The  Pathogenic  Bacteria  and  Protozoa" 

(8th  edition),  Phila,  1915.     Chapters  upon  Infection  and 

Immunity. 
JOSEPH      MCFARLAND:      Chapter      upon      "Immuno-Therapy," 

"Modern    Clinical    Medicine,"   volume    on    "Infectious 

Diseases,"  N.  Y.,  1910. 
H.  T.  RICKETTS:     "Infection,  Immunity,  and  Serum  Therapy," 

Chicago,  1906. 
VALLERIE-RADOT:     "The  Life  of  Pasteur." 


CHAPTER  XVI. 
MUTILATION  AND  REGENERATION. 

Regeneration  is  the  function  of  repair.  It  embraces  a 
number  of  dissimilar  processes.  Thus  certain  used-up 
or  worn-out  elements  are  constantly  renewed  and  in  this 
sense  repaired.  The  human  skin  is  subject  to  attrition 
by  which  its  superficial  cells  are  being  rubbed  off,  but 
new  cells  are  always  forming  to  take  their  place;  the 
nails  are  always  wearing  away,  but  are  always  growing 
from  the  matrix;  the  hairs  are  broken  or  shed,  but  con- 
tinue to  grow  or  new  hairs  to  take  their  place.  Among  the 
birds  there  are  periodical  moults,  when  the  old  feathers 
that  may  have  become  broken  or  useless  are  shed  and 
replaced  by  new  ones.  Reptiles,  lizards,  and  snakes 
periodically  shed  the  entire  skin  beneath  which  a  new 
one  has  formed.  Stags  annually  shed  their  horns, 
new  ones  of  slightly  different  form  being  produced  to 
take  their  places. 

When  the  entire  thickness  of  the  cuticular  covering  is 
accidentally  penetrated  and  the  subjacent  tissues  ex- 
posed, almost  every  living  organism  is  capable  of  effect- 
ing a  repair  by  which  the  exposed  surface,  if  not  too  large, 
becomes  covered  by  a  new  protective  skin.  When  the 
damage  is  of  a  more  serious  nature  and  a  part  of  the 
organism  torn  away,  the  reaction  that  follows  varies  in 
different  cases.  Sometimes  the  injury  simply  heals;  that 
is,  is  covered  by  a  new  protection  and  the  mutilation  per- 
sists; sometimes,  as  among  certain  ccelenterates  and 
worms,  there  is  a  rearrangement  of  the  remaining  tissues 
by  which  the  symmetry  of  the  organism  is  restored 
though  its  size  is  diminished;  and  sometimes  the  loss  of 
the  part  is  followed  by  a  new  growth  which  gradually 

401 


402  BIOLOGY:  GENERAL  AND  MEDICAL 

moulds  itself  into  the  exact  form  of  that  which  was  lost 
and  eventually  comes  to  perform  its  function.  So  the 
phenomena  of  regeneration  include  simple  healing,  the 
rearrangement  of  the  entire  organism,  or  the  restitution 
of  the  lost  part. 

It  is  easy  to  understand  that  cells  are  continually 
multiplying  in  the  rete  mucosum,  in  the  nail  matrix,  or  in 
the  hair  follicles,  undergoing  a  regular  series  of  trans- 
formations and  ending,  respectively,  in  horny  epiderm, 
nails,  and  hairs,  all  of  which  are  relatively  simple  struc- 
tures; but  when  a  peacock  moults  and  the  regenerative 
process  is  called  upon  to  produce  large,  elaborately  deco- 
rated, and  beautifully  colored  feathers,  each  of  which 
bears  a  definite  relation  of  size  and  figure  to  the  geomet- 
rically proportioned  pattern  of  the  bird's  spread  tail, 
one  cannot  help  feeling  that  something  more  than 
local  conditions  are  engaged  in  the  new  formation. 

Each  year  the  antlers  of  the  stag  are  shed,  but  grow 
again  as  soft  spongy  osseous  formations  which  become 
more  and  more  dense  or  eburnized  until  of  ivory  hardness. 
Each  year  the  antler  develops  along  new  lines,  increas- 
ing in  size  and  complexity  according  to  the  age  of  the 
stag,  always  in  conformity  with  the  type  of  the  species, 
but  never  jtwice  the  same  in  the  same  individual.  Here 
there  can  be  no  doubt  about  the  hereditary  character 
of  the  influences  controlling  the  regeneration. 

When  the  repair  following  injury  is  carefully  con- 
sidered, we  again  find  that  it  is  less  simple  than  at  first 
appears,  for  the  closure  of  the  wound  by  cicatricial  tissue 
and  its  covering  by  a  new  growth  of  the  ectoderm  is 
complicated  by  a  more  or  less  pronounced  tendency 
toward  the  renewal  of  those  parts  that  may  have  been 
destroyed  or  removed. 

The  present  knowledge  of  the  subject  is  insufficient 
to  enable  the  phenomena  of  regeneration  to  be  reduced 
to  orderly  scientific  principles;  we  no  doubt  confuse 
different  processes  with  one  another.  The  following 
arrangement  may  enable  the  student  to  appreciate 


MUTILATION   AND   REGENERATION 


403 


such  facts  as  are  known,  and  to  realize  the  difficulties 
in  the  way  of  accurately  comprehending  them. 

I.  The  mutilated  organism  restores  its  symmetry  by  a 
rearrangement  of  its  substance  and  recovers  its  size  by 
subsequent  growth. 

This  is  seen  in  lowly  organisms  only.  Thus,  when  the 
protozoan  Stentor  cceruleus  is  cut  transversely  into 


Fio.  143. — Stentor  cceruleus.  a,  Cut  into  three  pieces;  6,  regeneration  of 
anterior  piece;  c,  regeneration  of  middle  piece;  d,  regeneration  of  posterior  piece. 
(After  Gruber.) 

several  segments,  each  iragment  containing  nuclear  sub- 
stance, transforms  itself  into  a  more  or  less  perfect 
diminutive  of  the  original  in  the  course  of  a  few  hours  and 
is  then  ready  to  grow  to  its  normal  size.  Fragments 
without  nuclear  material  soon  die. 

A  similar  adjustment  is  seen  in  Hydra.  When  this 
organism  is  transversely  cut,  the  anterior  half  lengthens 
and  develops  a  new  base,  the  posterior  half  also  lengthens 
and  develops  new  tentacles,  so  that  two  new  hydras  are 
formed.  If  a  fragment  be  cut  out  of  the  centre  of  a 
hydra  by  two  parallel  transverse  incisions,  the  ends 


404 


BIOLOGY:  GENERAL  AND  MEDICAL 


gradually  close,  until  a  hollow  prolate  spheroid  is  formed. 
This  soon  becomes  more  and  more  prolonged  into  a 
cylinder,  at  one  end  of  which  tentacles  and  an  oral 
aperture  develop,  the  other  end  remaining  closed  to 
form  the  foot.  During  these  transformations  no  food 
is  consumed,  and,  therefore,  no  growth  is  possible;  the 
adjustment  must  be  accomplished  through  a  rearrange- 


FIG.  144. — Regeneration  in  Planaria.  a—e,  Planaria  maculata:  a,  normal 
worm;  b,  b1,  regeneration  of  anterior  half;  c,  cl,  regeneration  of  posterior  half ; 
d,  cross-piece  of  worm;  d1,  d2,  d3,  d4,  regeneration  of  same;  e,  old  head;  e1,  e-  e3, 
regeneration  of  same;  /,  Planaria  lugubris;/1,  regeneration  of  new  head  on  pos- 
terior end  of  same.  (After  Morgan.) 


ment  of  the  structures  already  present.  The  new  hydra 
thus  formed  soon  grows  to  the  normal  size,  however, 
after  feeding  again  becomes  possible. 

H.  V.  Wilson  found  that  when  simple  sponges  were  cut 
into  bits,  rubbed  up  and  passed  through  bolting  cloth, 
the  dissociated  cells  settling  to  the  bottom  of  the  water  in 


MUTILATION   AND    REGENERATION  405 

which  they  were  suspended,  gathered  themselves  together 
to  form  a  plasmodium,  which  after  a  brief  time  underwent 
structural  differentiation  eventuating  in  the  formation  of 
a  new  sponge.  Later,  working  in  the  same  manner  with 
certain,  hydroids,  he  found  that  when  treated  in  the  same 
manner — i.e.,  cut  up  and  squee/ed  through  bolting  cloth, 
— they  behaved  similarly  in  that  the  cells  first  clustered 
together  to  form  a  plasmodium,  and  later  underwent  dif- 
ferentiation with  the  reappearance  of  the  hydroid  polyp. 

Mutilated  planarians — unsegmented  worms — recover 
the  normal  form  after  mutilation,  by  the  twofold  process 
of  rearrangement  and  growth;  rearrangement  predominat- 
ing over  growth  if  the  organism  cannot  feed,  growth  pre- 
dominating over  rearrangement  if  it  can. 

It  goes  without  saying  that  this  form  of  regeneration 
is  only  possible  when  the  structure  of  the  organism  is 
relatively  simple.  So  soon  as  a  certain  degree  of  com- 
plexity is  reached,  it  ceases  and  multilation  results  either 
in  repair  or  in  the  restoration  of  the  lost  part  or  in  death. 

II.  The  mutilated  organism  grows  a  new  part  to  take 
the  place  of  that  which  has  been  lost. 

This  form  of  regeneration  is  interesting  because  it 
takes  place  through  influences  that  cannot,  at  present, 
be  clearly  understood. 

Morgan,  in  his  book  on  "Regeneration,"  makes  these 
divisions  of  the  subject: 

I.  Homomorphosis. — The  new  part  is  like  the  part 
removed. 

1.  Holomorphosis. — The  entire  part  is  replaced. 

2.  Meromorphosis. — The  new  part  is  less  than 
that  lost. 

II.  Heteromorphosis. — The  new  part  is  different  from 
that  removed. 

1.  The  new  part  is  a  mirror  figure  of  that  lost. 

2.  The  new  part  resembles  some  other  part 
than  that  lost. 

3.  The   new   part   is   unlike   anything  in   the 
body  (Neomorphosis). 


406  BIOLOGY:  GENERAL  AND  MEDICAL 

Other  descriptive  terms  used  by  Morgan  are  Epi- 
morphosis,  in  which  a  proliferation  of  new  material  pre- 
cedes the  development  of  the  new  .part,  and  Morpha- 
laxis,  in  which  a  part  is  transformed  directly  into  a  new 
organism  or  part  of  an  organism  without  proliferation  at 
the  cut  surfaces. 

So  far  as  is  known,  the  first  observations  upon  the 
regeneration  of  lost  parts  was  made  by  Bonnet,  who,  in 
1741,  experimented  with  earth-worms.  When  he  cut 
a  common  earth-worm  in  half,  the  anterior  half  grew  new 
segments,  forming  a  new  tail,  and  the  posterior  half 
new  segments  and  a  new  head,  so  that  eventually  two 
entire  worms  resulted.  The  regenerative  capacity  was 
not,  however,  so  restricted,  for  he  also  found  that  if  he 
cut  the  worm  into  three,  four,  eight,  ten,  or  even  fourteen 
pieces,  each  piece  eventually  reproduced  the  lost  seg- 
ments, including  the  head  and  the  tail,  so  that  as  many 
complete  worms  resulted  as  he  had  fragments  of  the 
original  worm.  "  The  growth  of  the  new  head  is  limited 
in  all  cases  to  the  formation  of  a  few  segments,  but 
the  new  tail  continues  to  grow  longer,  new  segments 
being  intercalated  just  in  front  of  the  end  piece  which 
contains  the  anal  opening."  "  Bonnet  found  that  if  a 
newly  regenerated  head  is  cut  off,  a  new  one  regenerates, 
and  if  the  second  one  is  removed,  a  third  new  one  de- 
velops, and  in  one  case  this  occurred  eight  times;  the  ninth 
time  only  a  budlike  outgrowth  was  formed."  "In  other 
cases  a  new  head  was  produced  a  few  more  times,  but 
never  more  than  twelve  times."  Short  pieces  removed 
from  either  end  of  the  worm  failed  to  regenerate,  but 
died  after  a  few  days.  Sometimes  two  new  heads  or 
two  new  tails  regenerated.  The  polarity  of  the  organ- 
ism was  always  preserved;  i.e.,  the  heads  always  grew 
at  the  anterior,  never  at  the  posterior,  end. 

These  results  of  Bonnet  have  been  confirmed  again 
and  ag«in.  The  regenerated  head  is  perfect,  including 
the  oral  opening,  the  oesophagus,  and  the  brain. 

Morgan  found  that  when  the  head  of  the  worm  known 


MUTILATION   AND   REGENERATION  407 

as  Allolobophora  fcetida  was  amputated,  its  regeneration 
was  always  perfect;  that  is,  if  one,  two,  three,  four,  or 
five  segments  are  removed,  exactly  the  same  number 
were  renewed.  If,  however,  six  or  more  were  removed, 
only  four  or  five  are  regenerated,  so  that  the  head  is 
perfected,  but  the  full  number  of  segments  behind  the 
head  is  never  reproduced.  He  found  this  to  be  the  rule 
for  all  the  annelids.  With  regard  to  the  posterior  end, 
he  found  that  when  it  was  amputated,  the  terminal  end 
contained  the  new  opening  of  the  alimentary  canal  and 
that  the  new  segments,  of  which  the  full  complement 
always  forms,  arise  in  front  of  this  terminal  segment,  the 
youngest  always  being  the  one  immediately  in  front  of  it. 

It  is  well  known  that  the  tails  and  fins  of  fishes  readily 
regenerate  when  multilated  or  amputated.  Morgan, 
in  his  lecture  before  the  Harvey  Society,  cited  an  experi- 
ment made  upon  the  Pacific  coast  for  the  purpose  of 
determining  whether  salmon  returned  from  the  sea  to 
the  same  rivers  in  which  they  were  born.  The  fish 
used  for  the  experiment,  thousands  in  number,  were 
marked  by  having  a  V-shaped  piece  cut  from  the  tail, 
but  as  the  tail  subsequently  regenerated  the  lost  part, 
the  markings  were  lost  and  the  experiment  failed. 

Spallanzani  (1768)  also  experimented  with  mutilated 
earth-worms,  confirming  what  Bonnet  had  found;  but 
went  further,  for  he  found  that  when  the  tail  was  cut 
from  a  tadpole  a  new  tail  grows  to  take  its  place.  If  the 
tadpole  is  fed,  it  grows  larger  while  the  tail  is  growing; 
if  it  is  not  fed,  it  ceases  to  grow,  but  a  new  tail  is  formed 
just  the  same.  Further  experiments  showed  that  sala- 
manders also  regenerated  amputated  tails,  including  the 
vertebrae,  and  that  if  the  leg  of  one  of  these  animals  was 
cut  off,  it  regenerated;  if  all  four  legs  were  amputated, 
all  four  legs  were  regenerated,  either  together  or  in  suc- 
cession as  they  were  removed.  The  regenerative  process 
proceeds  whether  the  animal  be  fed  or  not.  If  it  is  well 
fed,  it  grows  larger  and  the  lost  part  regenerates;  if  it 
is  not  fed,  it  grows  smaller,  but  the  leg  or  tail  continues 


408  BIOLOGY:  GENERAL  AND  MEDICAL 

to  regenerate  just  the  same.  It  takes  about  as  long 
for  the  perfect  regeneration  of  the  fingers  or  toes  as  for 
an  entire  limb.  If  a  limb  be  amputated  too  close  to  the 
body,  no  regeneration  takes  place,  though  the  wound 
heals.  In  one  experiment,  Spallanzani  amputated  all 
four  legs  and  the  tail  of  a  salamander  six  times  and  saw 
them  all  regenerate  six  times  during  the  three  summer 
months.  He  also  found  that  the  upper  and  lower  jaws  of 
salamanders  can  regenerate.  Lessona  found  that  terres- 
trial salamanders  cannot  regenerate  lost  parts,  though 
aquatic  species  of  the  same  genus  can  do  so.  Extend- 
ing these  experiments  still  further,  Spallanzani  found 
that  snails  can  regenerate  amputated  tentacles  and 
that  certain  of  them  can  regenerate  the  entire  head, 
collar,  or  foot. 

Since  the  time  of  Spallanzani  much  experimental 
work  has  been  done  and  many  facts  added  to  the  knowl- 
edge of  the  subject,  though  we  are  still  greatly  in  need 
of  illumination  concerning  the  general  principles  by 
which  what  is  known  can  be  correctly  correlated. 

We  now  know  that  lizards  frequently  lose  their  tails 
and  regenerate  them,  also  that  the  animals  seem  to  know 
that  they  can  do  so,  for  when  caught  they  unhesitat- 
ingly snap  them  off  to  escape.  Though  it  can  re- 
generate the  tail,  a  lizard  cannot  regenerate  the  limbs 
or  even  the  toes.  Newts  not  only  regenerate  the  tail 
and  limbs,  but  also  the  eyes.  Crustaceans — crabs  and 
lobsters — regenerate  legs,  fighting  claws,  and  sometimes 
antennae  and  eyes.  Certain  arthropods — myriapods, 
arachnids,  and  a  few  insects — are  able  to  regenerate 
lost  limbs,  but  this  power  is  restricted  to  a  few  species  of 
scattered  groups. 

In  all  of  these  cases  certain  facts  regarding  the  regen- 
erative power  must  be  noted.  Thus,  the  extent  of  the 
mutilation  determines  whether  the  injured  animal  shall 
die  or  live  as  well  as  whether  the  wound  shall  simply 
heal  or  shall  regenerate.  In  speaking  of  the  salamander's 
legs  it  has  already  been  remarked  that  though  they  may 


MUTILATION    AND    REGENERATION  409 

regenerate  many  times  in  succession,  if  the  amputation 
be  performed  too  close  to  the  body,  healing  without 
regeneration  results.  The  legs  of  crabs  and  lobsters 
regenerate  best  from  a  certain  point  known  as  the 
"breaking-joint,"  where  the  legs  are  constricted  and 
weakened  so  that  when  the  animal  is  caught  and  held  it 
not  infrequently  frees  itself  by  fracturing  the  leg  at  this 
point.  If  the  leg  be  broken  below  the  breaking-joint, 
the  animal  usually  rebreaks  it  at  that  point  and  casts 
aside  the  intervening  piece.  When  the  leg  is  amputated 
above  the  breaking-joint,  it  is  regenerated  with  greater 
difficulty.  Centipedes,  tarantulas,  and  walking-stick 
insects  are  found  to  have  "breaking- joints,"  and  these 
arthropods  regenerate  their  limbs  when  broken  there. 
Cockroaches  regenerate  the  tarsi,  but  not  the  leg  above 
the  tarsi. 

The  regenerative  power  appears  to  be  greater  in  pro- 
portion to  the  youth  of  the  animal.  Embryos,  larva?,  and 
young  animals  are  much  better  able  to  regenerate  lost 
parts  than  are  fully  formed  and  old  animals.  The  tad- 
pole may  regenerate  a  tail  or  a  limb,  but  the  frog  very 
rarely  and  imperfectly  regenerates  any  lost  member. 
When  legs  are  cut  off  of  caterpillars,  they  are  sometimes 
regenerated  during  the  pupa  stage,  so  that  the  imago 
has  the  full  complement. 

Temperature  has  a  marked  effect  upon  the  regenerative 
function.  Most  of  the  animals  possessed  of  regenerative 
powers  are  "cold-blooded,"  when  cold  they  are  inactive; 
when  warm  their  metabolic  functions  accelerate,  so 
that  if  they  are  kept  warm  or  the  experiments  performed 
in  the  summer  time,  regeneration  is  accelerated. 

Lastly,  complexity  of  structure  has  something  to  do 
with  the  regenerative  function. '  When  one  sees  that 
the  power  is  highly  developed  among  the  lower  verte- 
brates and  that  complexly  organized  members,  such  as 
salamanders'  legs  and  eyes,  can  be  correctly  reproduced, 
he  hesitates  to  dwell  upon  this  point.  Why  should  the 
lizard  regenerate  its  tail  and  not  its  legs;  why  should  the 


410  BIOLOGY:  GENERAL  AND  MEDICAL 

salamander  regenerate  its  tail,  legs,  and  eyes,  but  not  its 
head;  why  should  certain  birds  be  able  to  regenerate 
the  upper  mandible,  but  not  the  limbs?  These  are  diffi- 
cult questions  that  cannot  be  correctly  answered  in  the 
present  state  of  knowledge.  An  attempt  has  been  made 
to  regard  the  regenerative  function  as  a  matter  of  adapta- 
tion by  which  those  organisms  most  apt  to  be  mutilated 
have  become  equipped  for  the  emergency  by  an  unusual 
activity  of  the  reparative  function.  In  support  of  this 
theory,  the  breaking-joint  of  the  arthropod  leg  is  urged  as 
a  cogent  argument. 

Much  interest  attaches  to  the  nature  of  the  influences 
governing  the  reparative  process.  The  newly  formed 
part  usually  reproduces  the  lost  part,  but  sometimes  re- 
verses it.  Sometimes  a  mistake  is  made  and  a  wrong 
part  produced  as  when  attempted  regeneration  of  a 
crab's  eye  terminates  in  an  antenna-like  structure 
instead. 

Nothing  of  the  amputated  salamander's  hand  remains 
to  guide  the  growing  tissues,  yet  a  new  hand  complete  in 
all  its  parts  is  formed.  It  is  as  mysterious  as  the  phe- 
nomena of  heredity — yes,  more  so,  for  it  seems  more  easy 
to  conceive  that  the  ovum  contains  forces  which  by  acting 
and  reacting  upon  one  another  may  attain  to  a  finished 
product  than  that  that  product  once  finished  shall  be 
able  to  restore  itself  when  mutilated.  The  process  of  re- 
generation, however,  bears  every  evidence  of  being  domi- 
nated by  hereditary  influences,  for  that  which  grows 
upon  the  amputated  stump  of  the  salamander  is  a  sala- 
mander's limb,  not  a  lizard's  tail  or  a  mollusk's  eye. 
Spencer,  Darwin,  and  all  the  writers  upon  heredity  have 
found  it  necessary  to  include  the  phenomena  of  regen- 
eration among  those  for  which  heredity  must  account, 
and  see  in  it  additional  evidence  that  the  particular 
kind  of  physiological  units  to  which  they  attribute  the 
hereditary  influences  must  be  disseminated  throughout 
the  body. 

But  another  curious  fact  awaits  consideration.     If  the 


MUTILATION  AND  REGENERATION  411 

lost  part  be  replaced  by  a  similar  part  removed  from 
another  creature  of  the  same  kind,  the  regenerative 
function  is  inhibited.  The  new  part  is  accepted  in 
lieu  of  the  old  one,  grows  fast  by  the  process  of  healing, 
and  a  short  cut  to  the  desired  end,  the  restoration  of 
symmetry,  is  accomplished.  By  virtue  of  what  impres- 
sion is  the  suspension  of  regeneration  brought  about  in 
such  cases?  How  can  the  creature  or  any  of  its  parts 
know  that  it  need  not  grow  a  new  limb  because  some 
accident  has  already  furnished  one?  Why  is  it  satisfied 
with  one  ready  made  instead  of  making  the  new  one 
itself?  These  are  problems  difficult  of  solution,  the 
answers  to  which  may  never  be  known.  The  matter 
becomes  still  more  difficult  if  the  adaptation  theory  be 
entertained,  for,  granting  that  regeneration  be  an 
adaptation,  the  failure  of  regeneration  in  those  cases 
where  the  new  limb  is  substituted  for  the  amputated 
one  never  can  be  so  regarded,  seeing  that  the  imagina- 
tion can  scarcely  entertain  such  a  thought  as  that  of  mu- 
tilated animals  finding  adapted  parts  with  which  to 
replace  those  lost  and  so  doing  away  with  the  necessity 
of  preparing  them. 

Though  the  regenerative  phenomena  extend  through- 
out the  different  phyla  of  animals,  examples  being  found 
among  such  vertebrates  as  fishes,  batrachians,  reptiles, 
and  birds,  it  does  not  extend  to  the  mammals.  No 
authenticated  cases  are  on  record  in  which  parts  lost  by 
mammals  have  ever  been  regenerated.  These  highest 
and  most  complex  of  living  organisms  are  unhappy  in 
being  without  so  useful  a  function.  Among  them 
all  that  can  be  hoped  for  is  that  healing  may  follow 
injury. 

III.  The  Mutilated  Organism  Repairs  Itself  Without 
Restoring  its  Symmetry. — This  method  of  repair  has 
several  times  been  referred  to  as  "simple  healing." 
It  takes  place  through  proliferative  activities  of  the 
simpler  epithelial  and  connective  tissues.  It  also 
includes  restoration  of  a  few  damaged  tissues,  so  as  to 
be  regenerative  in  tendency. 


412  BIOLOGY:  GENERAL  AND  MEDICAL 

1.  Epithelial  Tissues. — Whenever  the  covering  epithe- 
lium is  removed  by  accident  or  destroyed  by  disease, 
repair  soon  begins  through  the  proliferation  of  cells  at 
the  periphery  of  the  denudation.     The  multiplying  cells 
extend  more  and  more  until,  if  the  denuded  area  is  not 
so  great  as  to  occasion  the  death  of  the  individual,  or  so 
infected  as  to  destroy  the  cells  as  they  form,  a  new  cover- 
ing is  produced.     This  new  integument  usually  lacks  the 
appendages  with  which  the  original  structure  may  have 
been  provided.     Thus  in  repair  of  the  skin,   the  hair 
follicles,  sweat  and  sebaceous  glands  are  usually  absent 
or  very  few,  and  in  the  mucous  membranes  the  glands 
are  absent.     The  type  of  epithelium  in  the  new  covering 
conforms    to    that    originally    present,    squamous    cells 
being  formed  where  squamous  cells  pre-existed,  columnar 
cells,  where  columnar  cells  pre-existed. 

2.  The  Fibriltar  Connective  Tissues. — When  the  injury 
or  disease  has  involved  a  greater  depth  of  tissue,  the 
fibrillar  connective  tissue  manifests  activity  and  soon 
shows  itself  to  be  the  most  important  factor  engaged 
in  the  process  of  repair.     Its  cells  multiply,  pass  through 
stages  analogous  to  those  seen  in  the  formation  of  the 
areolar  tissue  of  the  embryo,   and  eventually  produce 
fibres  of  collagen  and  fibroglia,  by  which  the  wound  is  at 
first  more  or  less  completely  closed  and  subsequently 
drawn  together.     Newly  formed  tissue  of  this  kind  is 
known  as  cicatricial  tissue  and  constitutes  the  "scar." 
It  at  first  appears  in  excess,  but  subsequently  contracts 
more  and  more,  loses  its  cellular  character,  and  becomes 
more  and  more  densely  fibrous  until  the  separated  edges 
of  the  wound  are  more  or  less  closely  approximated  and 
strongly  bound  together.     In  freshly  repaired  wounds 
one  sees  through  the  delicate  newly  formed  epiderm, 
a  mass  of  pink  scar  tissue  which  becomes  whiter  and  less 
conspicuous  as  time  elapses. 

3.  The   Blood    Vessels.— As  growing  tissues,  such   as 
form  the  new  scars,  require  to  be  nourished  during  the 
period  of  active  growth,  new  capillaries,  arterioles,  and 


MUTILATION   AND   REGENERATION  413 

venules  are  formed  to  meet  this  requirement.  Capil- 
laries are  formed  as  filamentous  offshoots  from  pre-exist- 
ing capillaries.  These  increase  in  size  and  gradually 
come  to  consist  of  several  endothelial  cells  which  become 
channeled.  Arterioles  and  venules  are  formed  by  en- 
largement of  capillaries  whose  walls  become  supported 
by  fibrillar  and  muscular  tissues  that  extend  over  them 
from  the  larger  vessels.  Such  new  vessels  may  be  per- 
manent or  may  be  of  temporary  use  only  and  sub- 
sequently disappear  through  the  pressure  exerted  upon 
them  by  the  contracting  fibrillar  tissue  as  the  repair 
becomes  more  and  more  perfect. 

4.  Bone. — In  all  animals  fractured  bones  are  per- 
fectly repaired  in  uncomplicated  cases.  As,  however, 
the  osseous  tissue  is  inelastic,  it  is  essential  that  the 
member  to  which  the  bone  belongs  shall  be  kept  abso- 
lutely quiet,  else  instead  of  a  bony  union,  only  a  fibrous 
union  will  take  place  and  a  false  joint  or  pseudarthrosis 
be  formed.  In  the  process  of  repair,  the  osteoblasts 
derived  from  the  periosteum  or  surrounding  membrane 
are  the  formative  cells.  They  first  elaborate  a  tem- 
porary or  provisional  tissue  of  a  nondescript  character, 
known  as  callus.  It  much  resembles  the  hyaline  carti- 
lage with  centres  of  ossification  seen  in  embryonal  bone 
formation,  and  as  it  calcifies  is,  like  it,  without  Haver- 
sian  systems  and  not  distinctly  laminated.  This  tissue 
is  the  crude  material  upon  which  the  bone  cells  subse- 
quently work  as  the  callus  is  reconstructed  and  rear- 
ranged so  as  to  bring  about  complete  continuity  of  the 
injured  bone,  after  which  the  surplus  is  removed.  The 
provisioned  callus  surrounds  the  ends  of  the  broken  bone 
with  a  spindle-shaped  mass  of  tissue  which  acts  the  part 
of  a  splint  until  the  true  or  definitive  callus  which  forms 
the  permanent  bond  of  union  is  formed,  after  which  it 
is  absorbed. 

The  union  of  the  bones  and  the  restoration  of  function 
usually  requires  but  a  few  weeks,  but  the  final  removal 
of  the  redundant  callus  and  the  restoration  of  the  symme- 
try of  the  bone  is  not  perfected  for  years. 


414  BIOLOGY:  GENERAL  AND  MEDICAL 

As  the  bone  is  a  product  of  the  periosteum,  the  loss  of 
much  bony  tissue  in  consequence  of  disease  is  not  in- 
compatible with  its  regeneration  if  the  periosteum  is 
not  destroyed  or  too  much  injured,  and  it  is  not  unusual 
for  surgeons  to  strip  off  a  fairly  healthy  periosteum  from 
a  diseased  bone,  remove  the  bone,  and  subsequently  find 
a  fair  substitute  manufactured  by  the  carefully  pre- 
served membrane. 

5.  Cartilage. — Damage  to  cartilage  is  usually  repaired 
by    the    intermediation    of    fibre-connective    tissue    by 
which  the  fragments  are  held  together,  no  new  cartilage 
being  formed. 

6.  Muscular  Tissues. — It  is  improbable  that  the  mus- 
cular  tissue   of   the  mammals  undergoes  any  effective 
regeneration   after  injury.     Wounds   of  the   unstriated 
muscle  of  the  uterus  and  intestines  repair  through  inter- 
mediate fibre-connective  tissue  cicatrices.     Wounds  of 
cardiac    and  voluntary  muscles   usually  do   the    same, 
though  peculiar  formations  sometimes   appear   at   the 
injured  ends  of  the  voluntary  muscle  fibres  which  many 
interpret  to  mean  that  regenerative  attempts  are  in  prog- 
ress.    However  this  may  be,  the  attempts  are  abortive, 
little  new  formation  results,  and  such  tissue  of  supposedly 
new  formation  as  may  be  found  at  the  ends  of  the  fibres  is 
distinctly  atypical. 

7.  The  Nervous  Tissues. — It  is  not  known  that  the  nerve 
cells  can  be  replaced  when  destroyed,   but  the  nerve 
fibres  regenerate  quite  well.     The  process  is  not  perfectly 
understood.     When  a  medullated  fibre  is  cut  or  torn  the 
proximal  end  degenerates  to  the  next  higher  "node  of 
Ranvier,"  and  that  of  the  distal  end  appears  to  degener- 
ate altogether.      If  there  is  no  infection  or    other  un- 
favorable condition  to  prevent  it,   the  regeneration  of 
the  nerve  begins  within  a  few  days  by  an  outgrowth 
from  the  proximal  end.     Such  outgrowths  from  the  axis 
cylinders  of  the  proximal  ends  grow  down  in  the  path 
of   the    medullary   sheaths,    extend    through    whatever 
cicatricial  tissue  may  be  in  process  of   formation,  and 


MUTILATION   AND   REGENERATION  415 

on  to  the  distal  fragment.  When  these  growing  axis 
cylinders  are  able,  as  the  result  of  a  neurotropic  influence, 
to  find  their  way  into  or  along  the  old  sheaths,  the  prog- 
ress toward  the  completion  of  the  conducting  tract  is 
rapid,  otherwise  it  is  slow  and  more  complicated  and 
perhaps  less  perfect  in  the  end.  Some  observers  deny 
that  the  distal  fragment  degenerates  completely,  but 
think  that,  like  the  proximal  end,  it  only  degenerates  a 
short  distance  so  that  the  growing  proximal  end  need 
not  renew  the  entire  path  of  conduction,  but  only  so 
much  as  has  been  destroyed.  Others  think  that  the 
axis  cylinder  fibre  is  restored  through  the  activity  of 
the  proliferated  cells  of  the  sheath  and  is  not  a  new 
growth  from  the  old  axis  cylinder. 

It  is  thus  seen  that  mammals  have  very  slight  powers 
of  regeneration,  though  some  evidences  of  the  new  for- 
mation of  important  tissue  elements  are  to  be  found  in 
most  cases  of  "simple  healing." 

IV.  Lost  Viscera  are  Regenerated. — There  can  be  little 
doubt  but  that  complexity  of  organization  plays  an 
important  part  in  this  event,  for  the  more  complexly  the 
organism  is  constructed,  the  greater  is  the  mutual 
dependence  and  indispensability  of  its  organs. 

To  an  organism  with  scarcely  any  viscera,  those  it  pos- 
sesses may  not  be  so  essential  that  life  may  not  be  easily 
maintained  for  a  considerable  time  without  them,  thus 
affording  opportunity  for  new  organs  to  form.  Such  a 
condition  is  seen,  for  example,  in  certain  sea-cucumbers, 
or  holothurians,  which  when  roughly  handled  eviscerate 
themselves,  yet  live  on  and  subsequently  regenerate  a 
new  set  of  organs.  Were  it  not  for  the  fact  that  the 
holothurian  can  live  for  a  long  time  without  organs,  it 
could  not  recover  from  the  injury.  In  all  cases  in  which 
an  organ  is  found  to  regenerate — brain  of  the  snail,  eye 
of  the  newt,  etc. — the  animal  must  be  able  to  dispense 
with  that  organ  during  as  long  a  time  as  its  regeneration 
necessitates. 

Less  careful  attention  seems  to  have  been  devoted  to 


416  BIOLOGY:  GENERAL  AND  MEDICAL 

this  phase  of  regeneration,  probably  because  of  the 
greater  difficulty  of  operating  upon  the  internal  organs 
of  small  animals. 

Among  the  vertebrates  there  is  very  little  true  regen- 
eration of  the  internal  organs.  If  a  kidney  be  removed, 
the  animal  lives  on  and  the  other  kidney  continues  to 
functionate  for  both,  increasing  in  size  for  the  purpose, 
not  by  the  formation  of  new  glomerules,  but  by  hyper- 
trophy, or  increase  in  the  size  of  those  already  present. 
In  cases  in  which  a  kidney  is  damaged,  by  operation  or 
disease,  new  tubules  have  been  found  to  bud  out  from 
the  pre-existing  tubules  and  to  extend  for  a  considerable 
distance  either  among  the  older  tubules  or  in  the  scar 
tissue,  but  there  is  no  new  formation  of  glomerules,  and 
hence  no  true  regeneration.  When  large  portions  of  the 
liver  are  removed  by  operation  or  destroyed  by  disease, 
the  remaining  portions  hypertrophy  to  carry  on  its  func- 
tion, and  not  infrequently  offshoots  from  the  bile  ducts 
are  found  extending  some  distance  into  the  cicatrices, 
as  though  new  liver  cell  columns  might  form,  but  the 
attempt  seems  to  be  abortive  and  to  include  only  the 
cells  of  the  ducts  and  not  those  of  the  parenchyma. 
The  removal  of  the  spleen  is  compensated  for  by  enlarge- 
ment of  other  lymphatic  organs  without  any  new  for- 
mation corresponding  to  the  splenic  structure. 

When  a  lung  is  removed  or  destroyed  by  disease,  no 
new  tissue  forms,  though  the  entrance  of  an  unusual 
quantity  of  air  may  cause  inflation  of  the  undisturbed 
tissue — an  injurious  rather  than  a  beneficial  effect. 

The  loss  of  the  heart  or  the  brain  is  certainly,  though 
not  immediately,  fatal.  Life,  however,  is  maintained 
under  these  circumstances  for  a  very  short  time  only  in 
most  cases.  Destruction  of  the  spinal  cord  results  in 
hopeless  palsy. 

As  the  phylogenetic  series  is  descended,  the  tenure  of 
life,  after  such  mutilations,  increases,  and  the  ability 
to  repair  damage  becomes  greater.  Thus,  in  man,  well 
authenticated  cases  of  regenerative  changes  in  the  eye 


MUTILATION   AND   REGENERATION  417 

are  rare,  but  in  the  triton  a  new  lens  is  easily  regenerated 
and  in  some  of  the  lower  batrachia  young  individuals 
may  regenerate  a  whole  eye.  Still  lower  animals  are 
capable  of  regenerating  the  head,  including  the  brain 
and  eyes,  but  the  organs  in  such  cases  are  simple  and  do 
not  form  counterparts  of  the  complex  brains  and  eyes 
of  the  vertebrates.  When  the  heart  is  a  simple  con- 
tractile tube  slowly  propelling  the  blood  through  vessels 
not  terminating  in  capillaries,  the  viscus  may  be  dis- 
pensed with  for  some  time,  during  which  a  new  one  may 
be  provided,  but  when,  as  in  the  vertebrates,  it  is  an 
elaborately  specialized  pump  with  complexly  arranged 
chambers  and  valves  and  when  the  somatic  life  is  main- 
tained solely  through  the  circulating  blood,  the  heart 
cannot  be  dispensed  with  at  all. 

REGENERATION  IN  PLANTS. 

This  subject  is  best  considered  under  two  separate 
headings:  1.  The  repair  of  damage;  2.  The  restoration 
of  lost  parts. 

1.  The  repair  of  damage  done  to  plants  is  effected 
through  changes  in  the  cells  injured  but  not  destroyed. 
The  destroyed  cells  die,  become  brown  and  dry,  and 
drop  off.  The  walls  of  the  underlying  cells  then  become 
lignified  or  wooden  and  the  more  delicate  cells  below 
thus  protected.  Such  changes  at  the  cut  edge  of  a  leaf 
protect  the  remainder,  which  lives  on  in  its  deformed 
state  for  a  long  time.  In  the  case  of  tubers,  as,  for  ex- 
ample, potatoes,  similar  changes  take  place  in  the  cut 
surfaces  and  thus  prevent  destruction  of  the  buds 
which  remain  alive,  so  that  cut  fragments  of  seed  potatoes 
may  be  kept  for  several  days  before  planting,  the  buds 
remaining  vital  and  beginning  to  grow  when  favorable 
opportunities  are  afforded. 

The  wooden  stems  of  higher  plants  when  super- 
ficially injured  are  repaired  by  an  active  growth  of  the 
living  cells  round  about  the  seat  of  injury,  forming  a 

27 


418  BIOLOGY:  GENERAL  AND  MEDICAL 

massive  development  of  what  is  called  callus  which 
gradually  extends  over  the  denuded  surface  until  it  is 
once  more  entirely  covered.  As  the  callus  grows,  the 
cells  become  suberized  and  a  cork-forming  phellogen 
arises  in  the  periphery.  In  the  stems  of  gymnosperms 
and  dicotyledons,  the  seat  of  injury  is  gradually  sur- 
rounded and  covered  by  a  layer  of  tissue  arising  from 
the  exposed  cambium  layer.  While  the  callus  is  gradu- 
ally spreading  over  the  wounded  surface,  an  outer  pro- 
tective covering  of  cork  is  formed,  at  the  same  time  that 
a  new  cambium  is  forming  within  the  callus,  through 
differentiation  of  the  inner  layer  of  cells  continuous  with 


FIG.  145. — Budding  leaf  of  Bryophyllum.     (From  Bergen   and   Davis'  "Prin- 
ciples of  Botany."     Ginn  &  Co.,  publishers.) 

the  cambium  of  the  stem.  When  the  margins  of  the 
growing  callus  meet  and  close  over  the  wound,  the 
edges  of  the  new  cambium  also  unite  and  form  a  complete 
cambial  layer  continuous  with  the  stem  and  covering 
the  entire  seat  of  injury  with  a  new  and  complete  cam- 
bium. The  new  wood  formed  by  this  new  cambium 
never  becomes  continuous  or  coalescent  with  the  old 
wood,  and  marks  that  cut  deeply  enough  into  the  stem 
to  penetjrate  the  wood  are  merely  covered  by  new  wood 
and  may  be  found  within  the  stem.  The  ends  of  sev- 
ered branches  may  similarly  become  so  completely 
covered  as  to  be  concealed  from  view.  The  callus  wood 
differs  in  certain  particulars  from  the  normal  wood,  con- 
sisting at  first  of  isodiametrical  cells  which  are,  how- 
ever, followed  by  the  formation  of  more  elongated 
cell  forms. 

2.  The  Restoration  of  Lost  Parts. — It  is  at  this  point 


MUTILATION   AND   REGENERATION  419 

that  regeneration  in  plants  and  animals  shows  the  great- 
est difference,  for  in  the  plant  no  regeneration  of  this 
kind  takes  place.  The  dissimilarities  between  animals 
and  plants  in  the  matter  of  growth  and  development 
throw  some  light  upon  the  subject,  for  nearly  all  plants 
grow  continuously  while  most  animals  reach  maturity 
and  subsequently  cease  active  growth.  Further,  in 
animals  the  germinal  matter  is  stored  up  in  the  gonads 
from  which  it  is  liberated  under  special  circumstances, 
while  in  vegetables  the  germinal  matter  seems  to  be 
widely  distributed  throughout  the  structure  and  merely 
concentrated  at  the  flowers  and  at  the  buds. 

This  wide  distribution  of  the  germinal  matter  makes 
it  more  easy  for  a  mutilated  plant  to  begin  life  anew  from 
one  of  the  germinal  buds  than  to  reconstruct  the  lost 
parts.  And  the  results  of  mutilation  show  this  to  be 
the  prevailing  tendency.  When  mutilation  is  effected, 
a  new  growth  starts  from  some  undisturbed  bud,  whether 
upon  leaf,  leaf-stalk,  bough,  branch,  trunk,  or  root,  and 
a  new  formation  occurs,  which  though  it  may  resemble 
and  serve  the  purpose  of  the  lost  part,  is  not  an  actual 
regeneration  as  is  the  new  tail  of  the  lizard  or  the  new 
limb  of  the  salamander.  It  is  rather  reproduction  than 
regeneration. 

The  capacity  for  such  new  growth  among  plants  varies 
greatly,  in  some  cases  seeming  to  be  almost  unlimited, 
as  in  the  willow,  of  which  almost  any  cut  fragment  stuck 
into  the  ground  will  take  root  or  almost  any  kind  of 
stump  sprout,  or  the  begonia  of  which  even  a  fragment 
of  a  leaf  will  sometimes  start  a  whole  plant. 

REFERENCE. 

THOMAS  H.  MORGAN:     "Regeneration,"  N.  Y.,  1901. 
H.  V.  WILSON:    Journal  of  Experimental  Zoology,  vol.  v.,  1907; 
vol.  ii.,  1911. 


CHAPTER  XVII. 
GRAFTING. 

By  grafting  we  understand  the  implantation  of  any 
portion  of  living  tissue  into  the  same  or  another  position 
in  the  same  organism,  or  into  the  same  or  a  different  posi- 
tion in  some  other  organism. 

The  results  are  found  to  vary  according  to  the  nature 
of  the  tissue  transplanted,  the  character  of  the  tissue 
into  which  it  is  transplanted,  the  ages  of  the  respective 
tissues,  the  physiological  importance  of  the  transplanted 
tissue,  the  physiological  necessity  the  organism  experi- 
ences for  it,  the  ability  of  the  transplanted  tissue  to 
maintain  itself  during  the  period  of  malnutrition  follow- 
ing the  transplantation,  and  the  blood-relationship  of 
the  respective  organisms  whose  tissues  are  concerned. 

The  general  facts  bearing  upon  grafting  apply  to  both 
the  vegetable  and  animal  kingdoms. 

1.  The  amputated  part  is  immediately  returned  to  its 
normal  environment. 

Under  these  circumstances  the  least  possible  amount 
of  disturbance  is  effected,  and  it  can  be  imagined  that 
if,  in  the  replacement  of  the  removed  tissue,  a  sufficient 
amount  of  care  is  exerted  in  approximating  the  tissues, 
there  is  no  essential  difference  between  such  an  opera- 
tion and  a  simple  incision.  Indeed  the  experiments  of 
Carrel  have  shown  that  the  chief  difficulty  is  in  restoring 
the  necessary  circulation,  and  that  if  this  can  be  success- 
fully overcome  by  end-to-end  anastomosis  of  the  blood 
vessels,  whole  limbs  may  be  removed  from  animals  as 
highly  and  complexly  organized  as  cats  and  dogs,  and 
successfully  replaced  or  even  exchanged.  With  the  cir- 

420 


GRAFTING  421 

culation  properly  maintained,  nothing  more  than  simple 
healing  is  required  to  restore  the  usefulness  of  the  part 
or  member.  When  the  tissue  fragment  is  too  small  to 
permit  of  vascular  suturing  and  must  temporarily 
derive  its  nourishment  by  imbibition  from  the  surround- 
ing tissues,  it  becomes  more  difficult  to  effect  transplan- 
tation of  considerable  masses.  Before  Nature  can 
provide  new  vessels  for  maintaining  it,  the  replaced  tissue 
commonly  dies  and  undergoes  mortification.  Tissues 
provided  with  free  capillary  plexuses  most  easily  survive, 
provided  they  are  not  composed  of  highly  specialized 
and  easily  damaged  elements.  In  injuries  of  the  human 
body  large  fragments  of  the  facial  tissues  torn  loose,  but 
not  entirely  away  from  their  attachments  may  be  success- 
fully replaced  if  not  seriously  infected.  Fingers  almost 
severed  or  torn  away  may  be  replaced,  and  in  a  few 
cases  fingers  entirely  cut  off  have  been  replaced,  carefully 
sutured,  and  have  successfully  united.  There  is,  how- 
ever, no  certainty  about  the  results  in  such  cases,  and 
the  surgeon  congratulates  himself  and  his  patient  when 
the  operations  terminate  in  recovery.  An  extracted 
tooth  restored  to  its  socket  will  again  grow  fast  and 
become  as  good  and  useful  as  before,  though  its  nutrition 
is  usually  imperfect.  New  blood  vessels  may  grow  into 
the  pulp-cavity,  and  new  nerve  fibres  find  their  way 
into  it. 

Among  the  higher  plants,  the  amputation  and  careful 
replacement  of  parts  in  such  a  cautious  manner  as  to 
secure  continuity  of  the  vascular  bundles  is  usually 
followed  by  the  continued  life  of  the  graft,  precautions 
being  taken  to  provide  artificial  support  until  firm  union 
of  the  fragments  has  been  secured. 

Among  the  lower  orders  of  animals  grafting  becomes 
correspondingly  easier  without  vascular  suture.  Thus 
when  the  tail  of  a  tadpole  or  the  leg  of  a  salamander  is 
removed  and  then  replaced,  some  means  being  provided 
for  holding  the  parts  in  place,  union  readily  takes  place 
and  the  usefulness  of  the  part  is  restored. 


422  BIOLOGY:  GENERAL  AND  MEDICAL 

In  these  cases  the  age  of  the  animal  experimented  upon 
plays  an  important  part  in  the  success  of  the  experiment, 
better  results  being  obtained  with  larval  than  with  adult 
organisms. 

Earth-worms  cut  apart  and  then  stitched  together 
readily  unite,  and  by  cutting  parts  from  several  worms 
and  stitching  them  together  unusually  long  worms  can  be 
made,  or  by  cutting  off  a  few  of  the  anterior  segments 
and  stitching  them  to  a  few  segments  cut  from  the  tail 
unusually  short  worms  can  be  produced.  This  reminds 
us  that  the  replacement  of  the  excised  part  inhibits  the 
process  of  regeneration.  When  the  leg  of  a  salaman- 
der is  cut  off,  a  new  leg  regenerates  as  has  been  shown; 
but  if  the  removed  leg  be  replaced  and  stitched  to  the 
stump,  it  grows  fast  and  no  new  leg  develops.  Similarly, 
if  the  tail  of  a  tadpole  be  amputated,  a  new  tail  regen- 
erates, but  if  the  amputated  tail  be  carefully  replaced 
it  grows  fast  and  no  regeneration  occurs.  If  in  replac- 
ing the  tail  the  work  be  done  carelessly  so  that  the 
coaptation  of  tail  and  body  be  imperfect  and  a  part  of 
the  stump  left  unprotected,  the  tail  may  grow  fast, 
but  from  the  uncovered  part  of  the  stump  a  new  tail 
grows,  so  that  the  animal  becomes  provided  with  two 
such  members. 

When  the  anterior  twenty  segments  and  the  posterior 
twenty  segments  of  an  earth-worm  are  amputated,  the 
former  regenerates  all  the  necessary  remaining  posterior 
segments;  the  latter  all  the  necessary  anterior  segments, 
and  the  middle  portion,  all  necessary  anterior  and  pos- 
terior segments,  so  that  three  complete  worms  eventually 
form.  But  if  the  anterior  twenty  segments  are  stitched 
to  the  posterior  twenty  segments,  the  two  portions 
grow  together,  no  intermediate  segments  are  regenerated, 
and  a  short  worm  is  formed  and  remains  short. 

Many  experiments  with  interesting  results  along  this 
line  were  made  by  Joest,  who  found  that  the  "satisfaction 
of  physiological  necessity"  did  not  seem  to  be  the  key  to 
the  situation,  seeing  that  there  might  be  two  sets  of 


GRAFTING 


423 


reproductive  organs  in  the  artificially  long  worms  and 
no  reproductive  organs  at  all  in  the  very  short  worms 
thus  produced. 

Should  one  endeavor  to  unite  two  worms  by  the  ante- 
rior ends  from  which  the  heads  have  been  removed,  or 
by  the  two  posterior  ends  from  which  the  tails  have 
been  cut  off,  difficulties  arise  that  indicate  the  strength 
of  the  force  of  polarity  among  living  organisms,  for 
though  union  may  occur,  a  head  often  springs  by  re- 


FIG.  146. — Heteroplastic  transplantation  in  the  earth-worm,  a,  Of  tail  end 
of  another  individual  of  the  same  species  (Allolobophora  terrestris) ;  b,  interca- 
lation of  mid-body  region  of  another  individual;  c,  lateral  grafting  of  another 
half  of  another  individual.  (Joest.) 


generation  from  the  seat  of  union  when  anterior,  or  a 
tail  or  a  head  when  posterior,  so  that  the  experiment 
eventuates  in  the  first  instance  in  two  worms  with  a 
head  in  common,  or  in  the  second,  two  worms  with  three 
heads,  or  two  worms  with  a  tail  in  common.  In  posterior 
sutures  the  regeneration  of  the  head  or  tail  seems  to 
depend  upon  the  length  of  the  amputated  portions.  If 
short,  a  tail  regenerates;  if  longer,  a  head  or  a  tail.  Mor- 
gan doubts  whether  Joest  is  correct  in  thinking  heads 
can  be  regenerated  from  combined  posterior  ends. 


424 


BIOLOGY:  GENERAL  AND  MEDICAL 


Much  experimental  manipulation  of  this  kind  has 
been  performed  with  hydras.  Trembly,  Watzel,  King, 
Morgan,  and  others  have  subjected  these  animals  to  a 
variety  of  amputations  and  abnormal  appositions,  and 


FIG.  147. — Regeneration  in  hydras,  a,  Hydra  split  in  two  hanging  vertically 
downward:  later  the  halves  completely  separated;  B,  two  posterior  ends  united 
by  oval  surfaces;  Bl,  same:  its  regenerated  two  heads,  each  composed  of  parts  of 
both  pieces.  B2,  absorption  of  one  piece  leading  to  a  later  separation  of  halves; 
C,  two  posterior  ends  united  by  oblique  surfaces:  later  one  piece  partially  cut 
off,  as  indicated  by  line;  C1,  later  still,  two  heads  developed,  one  at  M,  the  other 
at  N;  D,  similar  experiments  in  which  only  one  head  develops  at  M;  E,  five 
pieces  united  as  shown  by  arrows.  Four  heads  regenerated,  one  being  composed 
of  parts  of  two  pieces.  (Morgan  after  King.) 

found  that  the  force  of  polarity  is  easily  set  aside.  Thus 
the  organisms  can  be  made  to  .unite  either  by  their 
oral  or  aboral  ends.  Several  individuals  deprived 
of  the  oral  ends  and  tentacles  can  also  be  united  and 
very  long-bodied  individuals  produced.  In  one  case 


GRAFTING  425 

twenty-two  posterior  ends  were  united  and  then  one  of 
the  components  cut  in  two.  In  five  cases  a  single  head 
developed  upon  the  aboral  end  of  the  smaller  piece. 
When  several  pieces  are  united,  a  new  head  usually 
appears  at  the  line  of  juncture. 

Born  found  it  possible  to  make  transverse  sections 
through  tadpoles  and  then  reunite  them,  or  to  unite 
the  anterior  half  of  one  to  the  posterior  half  of  the  other. 
He  also  found  it  possible  to  unite  two  anterior  portions 
by  their  posterior  ends,  to  unite  them  dorsum  to  dorsum 
or  ventrum  to  ventrum,  and  in  the  latter  case  it  did 
not  matter  whether  they  were  placed  head  to  head  or 
head  to  tail. 

When  sections  passing  through  the  organs  were  made 
and  the  fragments  coaptated  organ  to  organ,  the  organs 
united  so  that  the  viscera  became  functional;  when  the 
coaptation  was  imperfect,  the  intervals  between  the 
organs  became  filled  in  with  connective  tissue,  and  the 
ability  of  the  animal  to  live  depended  upon  the  possi- 
bility of  enough  functional  activity  of  the  mutilated 
organs  being  retained. 

Ullmann  as  early  as  1902  transplanted  a  dog's  kidney 
to  its  neck,  united  the  renal  artery  with  the  carotid 
artery,  and  the  renal  vein  with  the  external  jugular  vein; 
the  end  of  the  ureter  being  stitched  to  the  skin.  The 
exact  results  of  this  experiment  are  uncertain.  The 
kidney  seems  to  have  remained  active  for  a  short  time, 
then  degenerated. 

Carrel  has  found  it  possible  to  remove  a  kidney  from 
a  dog,  perfuse  it  with  Locke's  solution  (a  physiological 
solution  used  to  wash  out  the  stagnated  blood  from  the 
vessels,  and  so  prevent  coagulation)  for  fifty  minutes, 
then  return  it  to  normal  environment  in  the  same 
animal,  anastomose  the  blood  vessels  and  nerves, 
and  have  the  organ  continue  its  function  almost  indefi- 
nitely. The  following  case,  one  of  five  experiments, 
will  serve  as  sufficient  proof:  "On  February  6,  1908, 
the  left  kidney  of  a  bitch  was  extirpated,  washed  in,  and 


426  BIOLOGY:  GENERAL  AND  MEDICAL 

perfused  with,  Locke's  solution,  and  replanted.  The 
circulation  was  re-established  after  having  been  inter- 
rupted for  fifty  minutes.  Fifteen  days  afterward  the 
right  kidney  was  removed.  The  animal  remained  in 
perfect  health.  In  June,  1909,  this  bitch  became  preg- 
nant and  gave  birth  to  eleven  pups.  In  December 
she  again  had  three  pups.  To-day,  twenty-three 
months  have  elapsed  after  the  operation,  and  she  is 
entirely  healthy." 

Guthrie  transplanted  the  ovary  of  a  pure  black  hen 
to  a  pure  white  hen  whose  ovary  had  been  removed. 
Subsequently  the  hen  laid  eggs,  and  upon  being  mated 
with  a  pure  white  cock,  laid  eggs  that,  upon  incubation, 
produced  white  and  black  chicks. 

2.  The  amputated  part  is  transplanted  to  a  new  environ- 
ment in  the  same  organism. 

Under  these  circumstances  the  conditions  are  some- 
what different,  for  though  the  general  physiologico- 
chemical  conditions  are  presumably  identical,  the  local 
conditions  vary.  It  must  not  be  imagined  that  the 
component  tissues  are  without  their  mutual  affinities 
and  repugnances,  for  were  there  no  such  influences  it  is 
difficult  to  conceive  how  the  organic  integrity  of  the 
complex  organisms  could  be  retained  in  cases  in  which 
accident  or  disease  bring  about  confusion  of  the  normal 
structure.  The  position  in  which  each  tissue  finds 
itself  as  the  result  of  ontogenetic  development  is  normal 
for  it,  and  under  normal  conditions  the  inherited  impulses 
of  the  cells  may  explain  the  preservation  of  the  inherited 
ontogenetic  relationship,  but  under  abnormal  conditions 
the  usual  disappearance  of  tissues  forced  into  abnormal 
relations  may  have  a  different  physiologico-chemical 
explanation;  that  is,  it  may  depend  upon  antagonistic 
actions  and  reactions  between  the  different  elements. 

Such  an  explanation  does  not  suffice  in  itself,  for  it 
is  only  possible  for  any  fragment  to  survive  transplanta- 
tion when  an  adequate  source  of  nutrition  can  be  found 
which  makes  it  almost  essential  that  the  transplanted 


GRAFTING  427 

tissue  in  order  to  survive  must  be  of  a  quality  that  can 
retain  life  in  spite  of  this  serious  handicap.  Few  tissues 
are  so  tenacious  of  life,  and  hence  very  few  survive 
transplantation.  Even  in  those  cases  in  which  the 
implanted  tissues  can  be  histologically  recognized  after 
an  interval  of  months,  they  are  found  to  be  decadent, 
and  in  practically  all  cases  they  are  destined  to  disappear. 

It  might  be  supposed  that  the  vitality  of  the  embryonal 
tissues  under  the  conditions  of  transplantation  would 
exceed  that  of  the  adult  tissues  because  of  their  greater 
cellular  activity,  general  capacity  for  growth,  and  ability 
to  live  upon  imperfectly  distributed  nourishment,  and 
this  is  true  for  it  is  quite  possible  to  effect  successful 
transplantations  in  such  embryos — salamander  larvae 
and  tadpoles — as  can  be  submitted  to  investigation, 
though  among  higher  vertebrates — reptiles,  birds,  and 
mammals — it  is  impossible. 

But  some  cases  of  extensive  transplantation  succeed. 
When  the  nose  is  lost  through  accident  or  disease,  sur- 
geons sometimes  build  up  a  new  nose  out  of  tissues 
obtained  from  the  patient's  finger.  The  tissue  of  the 
face  is  denuded  of  its  skin  over  an  area  of  appropriate 
size,  the  surface  of  the  chosen  finger  is  likewise  denuded, 
and  the  two  stitched  together.  Bandages  are  then 
applied  so  as  to  hold  the  hand  immovably  in  place  until 
firm  union  has  been  established  and  until  the  tissues 
of  the  finger  receive  some  new  vessels  from  the  face 
through  the  cicatrix.  When  the  surgeon  feels  confident 
that  this  has  been  effected,  the  finger  is  cautiously 
amputated,  and  its  tissues  so  manipulated  that  a  sem- 
blance of  a  nose  is  produced.  The  operation  commonly 
fails  because  of  the  great  difficulty  of  retaining  the 
finger  immovably  in  position  and  because  the  new  blood 
supply  afforded  the  digital  tissues  by  the  facial  vessels 
is  apt  to  be  inadequate. 

In  plastic  operations  of  this  and  similar  kinds,  where 
cutaneous  and  subcutaneous  tissues  are  transplanted, 
the  tissues  do  not  in  the  strict  sense  find  the  environ- 


428  BIOLOGY:  GENERAL  AND  MEDICAL 

ment  changed;  that  is,  the  fibrillar  tissue  meets  fibrillar 
tissue,  the  adipose  tissue  meets  adipose,  and  the  derm 
meets  derm. 

The  more  heterotropic  the  transplantations,  the  less 
the  probablilty  of  success.  Transplantation  is  not 
much  practised  in  human  surgery,  but  enough  experi- 
ments have  been  performed  upon  the  lower  animals  in 
the  laboratory  to  hold  out  considerable  hope  of  future 
success.  It  was  found  by  Hunter  and  Duhamel  that 
the  spur  of  a  young  cock  could  be  successfully  trans- 
planted to  its  comb  where  it  continued  to  grow  and 
eventually  attained  its  full  size. 

Ribbert  transplanted  the  mammary  gland  of  a  guinea- 
pig  a  few  days  old  to  a  position  upon  its  head  where  the 
graft  took  well  without  absorption.  The  animal  grew  up 
and  subsequently  bore  young,  and  it  is  interesting  to  note 
that  the  transplanted  mamma  secreted  milk  during  the 
period  of  lactation. 

Kocher,  in  1883,  transplanted  thyroids  in  dogs  with  a 
certain  amount  of  success;  and  Schiff,  in  1884,  obtained 
temporary  benefit  and  the  prevention  of  cachexia 
strumipriva  in  human  beings  by  grafting  thyroid  tissues 
after  removal  of  the  thyroid  gland  for  disease. 

Von  Eiselberg  (1892)  transplanted  one-half  of  a  cat's 
thyroid  into  its  abdominal  wall,  waited  until  the  wound 
had  healed,  and  then  transplanted  the  other  half  into 
the  abdominal  wall  or  cavity.  The  animal  bore  the 
operation  well  and  lived  on,  the  grafts  remaining.  When 
the  grafts  were  later  excised,  tetany  quickly  developed, 
and  the  animal  died.  These  experiments  show  that 
the  thyroid  is  able  to  persist  and  functionate  in  a  new 
environment. 

The  experiment  has  since  been  repeated  many  times, 
and  it  is  now  certain  that  the  transplanted  thyroid  can 
remain  functional  during  the  entire  remainder  of  the 
animal's  life. 

The  success  or  failure  of  the  transplantation  seems  to 
depend  in  large  measure  upon  the  physiological  necessity 


GRAFTING  429 

for  the  transplanted  tissue.  Thus,  if  a  fragment  of  the 
thyroid  is  transplanted  and  the  greater  part  permitted 
to  remain  in  its  normal  position,  the  graft  is  apt  to 
suffer  absorption — apparently  because  it  is  not  physio- 
logically necessary.  The  absence  of  the  physiological 
necessity  probably  explains  many  failures  in  trans- 
planting thyroid  tissue. 

Knauer  and  Grigorieff  performed  many  experiments 
by  transplanting  the  ovaries  of  rabbits  to  new  situations 
in  the  abdominal  cavity  and  found  that  though  the 
central  portions  of  the  transplanted  organs  usually 
underwent  necrosis  and  were  replaced  by  fibre-connect- 
ive tissue,  the  superficial  layer  containing  the  follicles 
and  ovules  escaped  destruction  so  that  the  function  of 
ovulation  was  not  affected.  Three  of  the  rabbits  whose 
ovaries  had  thus  been  transplanted  subsequently  became 
pregnant,  so  that  it  appears  that  the  ovules  liberated  into 
the  abdominal  cavity  found  their  way  to  the  Fallopian 
tubes  and  uterus. 

Morris,  in  removing  the  ovaries  and  tubes  from  a  human 
patient,  grafted  a  portion  of  one  of  the  removed  ovaries 
upon  the  stump  of  one  of  the  tubes.  This  graft  took, 
and  the  patient  later  became  pregnant. 

Skin  grafting  has  become  a  frequent  and  useful  surgi- 
cal method  for  facilitating  the  restoration  of  the  dermal 
covering  in  extensive  ulcerations  such  as  follow  super- 
ficial burns,  etc.  The  grafts  can  be  taken  from  any 
part  of  the  patient's  body  and  need  not  be  large;  in  fact, 
a  number  of  small  grafts  seem  quite  as  useful,  if  not 
more  so,  than  the  transplantation  of  considerable  por- 
tions of  skin.  In  these  cases,  the  superficial  layers  of 
the  epiderm  are  useless  as  they  have  no  longer  sufficient 
vitality  to  permit  them  to  multiply;  any  transplanted 
subcutaneous  tissue  is  likewise  useless,  as  it  is  absorbed. 
The  essential  portion  comprises  the  rete  mucosum  whose 
cells  have  great  tenacity  of  life — they  may  be  kept 
alive  in  salt  solution  for  ten  days  or  two  weeks — become 
amoeboid  in  the  new  environment,  and  by  their  multi- 


430  BIOLOGY:  GENERAL  AND  MEDICAL 

plication  and  rearrangement  form  the  new  epithelial 
covering. 

Certain  of  the  mucous  membranes  are  also  able  to 
survive  transplantation,  and  good  results  have  followed 
plastic  operations  in  which  fragments  of  tissue  from 
the  mouth  have  been  used  to  assist  in  the  restoration 
of  destroyed  conjunctiva. 

3.  The  amputated  part  is  transplanted  to  another  animal 
or  plant  of  the  same  kind. 

Under  such  circumstances  the  new  environment  to 
which  the  graft  is  transplanted  differs  from  that  of  the 
autoplastic  grafts  in  so  far  as  the  physiological  condi- 
tions of  two  individuals  may  differ.  As  has  been  shown 
in  the  chapter  upon  Blood  Relationships,  the  chemical 
and  physiological  conditions  among  individuals  of  the 
same  species  are  usually,  but  not  necessarily,  identical. 
When  they  happen  to  differ,  the  grafts  may  fail  exactly 
as  when  the  grafts  are  heteroplastic. 

Were  it  not  for  physiologico-chemical  variation,  this 
form  of  transplantation  might  be  looked  upon  as  the 
future  hope  of  surgery,  for  Carrel  has  shown  its  extraor- 
dinary possibilities.  Thus  he  has  removed  the  kidneys 
of  a  cat  and  replaced  them  by  the  kidneys  of  another 
cat.  The  animal  recovered  from  the  operation  and 
lived  on  in  apparent  health  for  some  time.  He  has 
also  transplanted  a  limb  from  one  dog  to  another  dog,  and 
removed  most  of  the  tissues  from  one  side  of  the  face  of 
one  dog  to  the  corresponding  situation  upon  another  dog. 

These  results  justify  the  hope  that  the  time  may  not 
be  far  distant  when  normal  kidneys  from  a  normal  person 
killed  by  accident  may  be  implanted  into  the  body  of 
another  whose  kidneys  are  diseased,  and  that  a  part  of  a 
limb  amputated  for  traumatic  injury  or  taken  from  a 
person  suddenly  killed  by  accident  may  be  used  to 
supply  a  limb  needed  by  some  other  person  whose  mem- 
ber is  lost  through  disease.  Indeed,  Lexer  has  thus  trans- 
planted a  knee-joint  from  one  man  to  another  with 
success. 


GRAFTING  431 

The  difficulties  in  the  way  of  blood-vessel  anastomo- 
sis have  been  overcome,  but  the  physiologico-chemical 
difficulties  remain,  and  when  these  experimental  trans- 
plantations are  carefully  scrutinized  it  is  found  that 
sooner  or  later  the  experiment  animal  is  apt  to  die 
because  of  some  condition  referable  to  them.  This 
is  well  exemplified  in  the  transplantation  of  a  cat's 
kidney  by  Carrel  and  Guthrie.  One  kidney  of  a  healthy 
cat  was  removed  and  replaced  by  the  healthy  kidney  of 
another  cat.  The  animal  recovered  perfectly  from  the 
operation  and  lived  about  a  year,  when  her  previously 
undisturbed  kidney  was  removed.  After  this  operation 
she  died  in  a  few  days  with  the  usual  symptoms  of  renal 
insufficiency.  Upon  examination  with  the  microscope  it 
was  found  that  the  ingrafted  kidney  had  suffered  histo- 
logical  changes  that  made  it  unable  to  functionate. 
It  is  also  exemplified  in  Carrel's  case  of  successful  trans- 
plantation of  both  kidneys  of  a  cat  where  it  was  subse- 
quently found  that  the  aorta  and  blood  vessels  had 
undergone  an  extraordinary  calcification  unlike  any- 
thing previously  known  to  take  place  in  cats,  and  in  some 
way  directly  or  indirectly  referable  to  the  changed 
physiological  conditions  associated  with,  or  following 
the  operation. 

It  is  not  uncommon  for  a  person  to  donate  a  sound 
front  tooth  to  another  whose  tooth  is  extracted  as 
worthless.  Such  a  sound  tooth  may  be  implanted  in 
lieu  of  that  lost,  grows  fast,  and.  remains  useful  for  a 
long  time.  Here,  however,  the  conditions  are  somewhat 
different,  for  the  tooth  implanted,  though  it  remains  in 
place  and  is  firmly  attached  and  functionally  useful,  is 
commonly  a  dead  tooth  and  would  quickly  slough  away 
if  it  were  soft  tissue.  What  applies  to  the  recently  ex- 
tracted tooth  applies  equally  to  teeth  extracted  long  be- 
fore or  to  teeth  soaked  in  antiseptic  solutions  or  to  bits 
of  ivory  fashioned  into  resemblance  to  teeth  or  to  artifi- 
cial teeth  made  of  bone.  All  such  grow  fast  and  remain 
useful  for  considerable  lengths  of  time,  according  to  their 


432  BIOLOGY:  GENERAL  AND  MEDICAL 

power  to  resist  the  external  forces  with  which  they  have 
to  contend. 

The  less  imperative  the  nutritional  requirements  of 
any  tissue,  the  more  apt  it  is  to  withstand  absorption 
in  the  new  environment.  While  it  persists,  new  tissue 
may  grow  in  and  about  it  so  that  when  its  final  disap- 
pearance takes  place  it  may  not  be  missed.  In  some 
cases  the  transplanted  tissue  survives  exactly  as  in 
autoplastic  operations.  Thus  it  is  that  in  Carrel's  experi- 
ment upon  the  replacement  of  a  blood  vessel  by  the  em- 
ployment of  a  part  of  a  vessel  from  another  animal,  the 
transplanted  fragment  is  able  to  perform  its  function 
even  though  it  be  kept  on  ice  or  otherwise  for  some 
days  before  being  put  in  place. 

The  inherent  vitality  of  any  tissue  has  something  to 
do  with  its  ability  to  persist  after  transplantation,  the 
differences  in  different  tissues  being  shown  by  the  experi- 
ments of  Ribbert  who  transplanted  a  variety  of  different 
tissues  to  the  lymph  nodes.  Epithelial  cells  so  trans- 
planted shortly  died;  fragments  of  salivary  glands  per- 
sisted for  a  longer  time,  the  glandular  cells  changing 
to  a  cuboidal  type,  and  the  duct  epithelium  becoming 
flat;  liver  tissue  so  transplanted  underwent  a  central 
necrosis,  but  the  surface  remained  alive  for  some  weeks 
until  the  epithelial  cells  were  destroyed  by  the  com- 
pressing effect  of  the  connective  tissue;  kidney  tissue 
was  so  transformed  that  the  cells  of  the  convoluted 
tubules  came  to  resemble  those  of  the  straight  tubules 
and  the  tissue  to  resemble  that  of  the  kidney  of  chronic 
interstitial  nephritis,  after  which  it  was  gradually  ab- 
sorbed; when  the  skin  was  transplanted  in  such  manner 
that  both  the  epiderm  and  cutis  were  included  in  the 
graft,  the  cells  continued  to  be  nourished  by  their  sub- 
jacent tissue  and  spread  out  until  they  lined  the  space 
into  which  the  tissue  was  transplanted  and  a  cyst  was 
formed.  The  transplantation  of  connective  tissues  some- 
times fails,  sometimes  succeeds.  The  softer  the  tissue, 
the  sooner  it  is  absorbed ;  the  denser  the  tissue,  the  longer 


GRAFTING  433 

it  persists  and  the  more  likely  is  the  graft  to  grow.  Thus, 
transplanted  fragments  of  perichondrium  and  per- 
iosteum not  infrequently  remain  and  produce  new  cartil- 
age and  new  bone.  The  formation  of  new  bone  by  the 
periosteum  can  only  be  effected,  however,  when  the  cells 
have  some  bony  tissue  to  work  upon,  so  that  if  it  is 
desired  to  produce  bony  formation  by  transplanted  per- 
iosteum, it  is  necessary  to  add  fragments  of  bone,  pref- 
erably fragments  to  which  the  periosteum  is  already 
attached. 

Morris  successfully  grafted  a  part  of  an  ovary  from 
one  woman  upon  the  uterine  wall  of  another  whose 
ovaries,  being  diseased,  were  removed.  The  patient 
subsequently  menstruated,  showing  that  the  graft  not 
only  lived,  but  performed  a  vicarious  function. 

In  surgical  skin  grafting  it  is  not  unusual  for  one 
normal  person  to  donate  some  of  his  skin  to  supply  an- 
other with  needed  integument.  Just  as  in  autoplastic 
grafting,  such  grafts  usually  take,  though  whether  the 
newly  acquired  skin  persists  or  is  gradually  absorbed 
and  replaced  by  skin  of  the  patient's  own  development 
is  a  question,  for  interesting  changes  take  place  in  the 
graft. 

Thus,  when  the  skin  of  a  negro  is  grafted  upon  a  white 
person,  it  remains  for  a  time  unchanged,  then  either 
loses  its  pigment  or  is  thrown  off  by  a  new  white  skin 
that  develops  beneath  it.  Loeb  found  that  when  skin 
from  a  colored  guinea-pig  was  transplanted  to  an  albino, 
it  eventually  lost  its  color  and,  vice  versa,  when  the 
skin  of  an  albino  was  transplanted  to  a  colored  animal, 
it  became  pigmented  in  the  course  of  time.  Here, 
again,  we  cannot  be  certain  that  the  transplanted  skin 
persists.  It  may  be  imperceptibly  destroyed  and 
replaced  by  the  gradual  growth  of  the  normal  skin  of  the 
animal. 

The  transplantation  of  embryonal  tissues  is  not  differ- 
ent from  that  of  adult  tissues.  Fischer  transplanted  the 
leg  of  an  embryo  bird  to  the  comb  of  a  cock  or  a  hen,  and 
28 


434  BIOLOGY:  GENERAL  AND  MEDICAL 

found  that  it  grew  fast  and  appeared  like  a  successful 
graft,  but  changed  after  a  few  months,  degenerated,  and 
was  cast  off. 

Thus  it  appears  that  with  the  exception  of  those  cases 
in  which  the  organism  as  a  whole  experiences  the 
"physiological  necessity"  for  the  engrafted  tissue,  the 
general  tendency  is  for  the  graft  to  slowly  change  and 
disappear. 

4.  The  amputated  part  is  transplanted  to  another  animal 
or  plant  of  a  different  kind. 

Here  we  are  confronted  by  the  theoretical  and  practi- 
cal difficulties  arising  from  the  physiologico-chemical 
divergences  existing  among  different  species,  genera, 
families,  orders,  phyla,  etc.  In  most  cases  these  can 
be  prejudged  by  the  anatomical  differences,  but  there 
are  exceptions. 

From  the  facts  at  our  disposal  we  are  now  able  to 
state  that  the  closer  the  blood-relationship  of  the  organ- 
ism furnishjng  the  graft  and  the  organism  receiving  it, 
the  more  probable  the  success  of  the  experiment. 

The  experiments  thus  far  reviewed  have  shown  that 
very  slight  differences,  even  such  as  arise  among  indi- 
viduals of  the  same  species,  may  interrupt  the  successful 
progress  of  tissue  implantations  and  suggest  that  the 
greater  differences  between  individuals  of  different 
species  may  entirely  prevent  them.  Let  us  see  how 
these  theoretical  suggestions  are  borne  out  by  the  facts 
obtained  by  experiment. 

We  have  already  seen  that  hydras  are  susceptible 
of  experimental  manipulations  of  many  kinds  and  can  be 
successfully  grafted  together  in  many  different  ways. 
When  the  conjoined  hydras  are  of  different  species, 
however,  the  results  are  different;  thus  Wetzel  conjoined 
Hydra  fusca  and  Hydra  grisea  and  observed  complete 
union  in  five  hours.  But  later  a  constriction  appeared 
where  the  fragments  had  been  united,  the  head-piece 
produced  a  foot  near  the  line  of  union,  and  the  lower 
end  produced  a  circle  of  tentacles.  After  eight  days 


GRAFTING 


435 


the  organism  was  killed  for  examination,  and  fell  apart 
in  two  pieces — evidently  the  primary  union  was  tempo- 
rary, and  being  unsuccessful  was  followed  by  regeneration. 
In  Joest's  experiments  with  earth-worms  it  was  found 
to  be  difficult,  though  possible  to  successfully  unite 
Lumbricus  rubellus  and  Allolobophora  terrestris  so 
that  a  single  individual  was  produced  that  lived  for 


FIG.  148.— The  upper  figure  shows  two  tadpoles  of  Rana  esculenta  conjoined 
by  the  dorso-cephalic  surfaces. 

In  the  lower  series  A  shows  Rana  esculenta  and  Rana  arvalis  successfully 
grafted  upon  one  another;  B,  Bombinator  igneus  and  Rana  esculenta  success- 
fully grafted,  and  C,  the  anterior  half  of  Rana  esculenta  successfully  engrafted 
upon  the  posterior  half  of  Rana  arvalis.  (Redrawn  from  Born.) 

eight  months.     Each  piece  is  said  to  have  retained  its 
specific  characteristics  without  modification. 

Born,  in  his  experiments  upon  tadpoles,  found  it 
possible  to  unite  parts  of  animals  of  different  species, 
and  even  of  different  genera;  thus  in  one  experiment  he 
was  successful  in  securing  a  union  comprising  an  anterior 
half  of  Rana  esculenta  with  the  posterior  half  of  Bombi- 


436  BIOLOGY:  GENERAL  AND  MEDICAL 

nator  igneus.  After  ten  days,  however,  pathological 
conditions  were  observed  and  the  animal  was  killed  for 
further  and  minute  study. 

Another  combination  consisted  of  an  anterior  part 
of  Rana  esculenta  and  a  posterior  part  of  Rana  arvalis 
and  lived  for  seventeen  days.  Each  half  in  both  of 
these  experiments  retained  its  specific  characters,  though 
the  circulation  was  common. 

Harrison  had  still  better  results  for,  having  com- 
pounded an  organism  of  portions  of  the  tadpoles  of  Rana 
virescens  and  Rana  palustris,  he  was  able  to  keep  it  alive 
until  it  changed  into  a  frog,  each  half  of  which  continued 
to  show  its  own  specific  characters. 

Crampton  succeeded  in  effecting  coalescence  of  two 
lepidopterous  pupae  (Callosamia  promethia  and  Samia 
cecropia),  by  fastening  the  respective  portions  together 
with  melted  paraffine.  The  point  of  union  in  the  subse- 
quently developed  imago  resembled  the  hairless  bands 
that  connect  the  abdominal  segments. 

The  results  of  transplantations  effected  in  the  higher 
animals  are  usually  what  might  be  predicted.  The  im- 
planted part,  if  superficial,  sloughs  off;  if  deep,  is  absorbed. 
Bert  transplanted  the  tail  of  a  white  rat  to  the  body 
of  Mus  decumanus  where  it  remained  alive;  he  failed 
when  he  tried  to  graft  the  tail  of  a  field  mouse  upon 
a  rat,  and  he  had  no  success  in  his  attempts  to  graft 
the  tail  of  a  rat  upon  the  body  of  a  dog  or  a  cat. 

As  has  been  shown,  when  the  skin  of  a  negro  is  grafted 
upon  a  Caucasian,  the  pigment  in  the  skin  disappears, 
and  eventually  all  trace  of  the  graft  is  lost  if  it  is  not 
exfoliated  en  masse. 

The  transplantation  of  sheeps'  thyroids  into  human 
beings  in  cases  in  which  the  thyroid  is  functionless  has 
been  performed  with  temporary  relief,  but  absorption 
of  the  implanted  tissue  usually  takes  place  even  though 
the  "physiological  necessity"  that  is  favorable  to  suc- 
cessful grafting  seems  to  be  present. 

For  many  years  pathologists  have  been  industriously 
experimenting  in  the  hope  of  arriving  at  some,  definite 


GRAFTING  437 

knowledge  of  the  nature  and  cause  of  tumors.  Being 
unable  to  apply  the  cultivation  methods  with  success, 
and  still  expecting  to  find  an  infectious  agent  by  which 
to  account  for  these  formations,  they  made  many  series 
of  experiments  by  implanting  fragments  of  tumors 
derived  from  human  beings  into  various  tissues  and 
cavities  of  the  lower  animals.  The  literature  upon 
the  subject  is  large  and,  when,  reviewed,  shows  that 
great  ingenuity  and  the  utmost  precaution  have  been 
employed  that  the  implantations  should  be  made  under 
the  most  favorable  conditions.  Shattuck  and  Ballance, 
in  order  that  nothing  might  be  neglected  that  would 
contribute  to  success,  even  went  to  the  length  of  intro- 
ducing entire  human  mammary  carcinomas  (cancers) 
into  the  abdominal  cavities  of  sheep  and  other  animals. 
In  every  case,  irrespective  of  the  precautions,  such  tumor 
transplantations  failed,  and  the  conclusion  was  about 
reached  that  no  tumor  tissue  of  any  kind  could  be  suc- 
cessfully transplanted,  when  Hanau,  in  Weigert's  labo- 
ratory, came  into  possession  of  a  rat  with  a  squamous 
cell  carcinoma  of  the  vulva,  which  he  transplanted  to 
other  rats.  To  his  and  everybody's  surprise  these 
grafts  grew,  developed  into  tumors,  behaved  like  spon- 
taneous tumors,  and  caused  the  formation  of  metastatic 
tumor  nodules  in  the  lymph  nodes.  The  experiment 
was  for  a  long  time  conspicuous  because  of  its  exceptional 
results;  then  Jensen  transplanted  a  tumor  of  a  white 
mouse  to  other  white  mice,  and  was  successful.  The  so- 
lution of  the  problem  was  eventually  found  in  the  homol- 
ogous and  heterologous  nature  of  the  transplantations. 
Heterologous  grafting  results  in  the  disappearance  of 
the  graft  by  absorption;  homologous  grafting — trans- 
plantation of  the  tissue  to  other  animals  of  the  same 
kind — may  be  successful,  and  many  investigators  in  dif- 
ferent parts  of  the  world  have  since  been  able  to  achieve 
and  continue  the  homologous  transplantation  of  mouse 
and  rat  tumors  for  indefinite  lengths  of  time  through 
hundreds  of  generations.  When  heterologous  trans- 


438  BIOLOGY:  GENERAL  AND  MEDICAL 

plantations  have  succeeded,  *as  in  the  case  of  the  trans- 
plantation of  the  Jensen  rat  sarcoma  to  the  developing 
chick  embryo  by  J.  B.  Murphy,  the  explanation  may 
possibly  be  found  in  the  comparatively  undifferentiated 
condition  of  the  embryonal  tissues,  for  the  same  tissue 
died  at  once  when  transplanted  to  the  adult  fowl,  even 
after  several  generations  of  growth  in  the  embryo  chick. 
Unfortunately,  these  successes  have  not  yet  thrown  as 
much  light  upon  the  etiology  of  tumors  as  they  have  on 
the  matter  of  blood  relationship  and  tissue  affinities. 
We  have  confirmed  the  fact  that  the  tissues  of  different 
animals  will  not  agree,  but  we  have  not  learned  the  nature 
of  tumors. 

The  question  may  be  asked  why  the  tumor  tissue 
behaves  differently  from  the  normal  tissue  when  trans- 
planted, for  we  remember  that  even  in  the  homologous 
and  autc plastic  transplantations  of  normal  tissues  the 
grafts  ar3  usually  subject  to  decadence,  death,  and  ab- 
sorption. The  answer  seems  to  be  found  in  the  abnor- 
mal impulse  of  growth  that  characterized  the  tumor 
tissue  and  stamps  it  as  such.  It  is  tissue  that  would 
grow  unrestrictedly  in  its  normal  environment,  and 
continues  to  do  so  when  transplanted. 

When  we  come  to  consider  the  conditions  of  successful 
grafting  in  the  vegetable  world  we  find  that  with  certain 
exceptions  they  form  a  parallel  with  what  has  already 
been  found  in  the  animal  world.  That  is,  their  success 
or  failure  depends  chiefly  upon  the  blood  relationship 
of  the  plants  concerned,  though  this  restriction  is  not 
so  closely  defined  as  among  animals.  Plants  of  the 
same  species  can,  other  things  being  equal,  easily  be 
grafted  upon  one  another;  plants  of  the  same  species, 
but  of  different  varieties  can  usually  be  grafted  one  upon 
the  other;  plants  of  different  species  can  sometimes  be 
grafted  one  upon  the  other;  plants  of  different  genera  can 
rarely  be  grafted  upon  one  another,  and  after  the  generic 
line  is  past,  attempts  made  to  graft  individuals  of  differ- 
ent families  and  orders,  invariably  fail. 


GRAFTING 


439 


Grafting  among  plants  has  been  practised  from  an- 
tiquity. How  the  idea  originated  or  why  it  was  origi- 
nally practised  is  unknown.  To  grafting,  however,  the 
ancients  attributed  results  of  kinds  not  borne  out  by 
modern  scientific  examination.  Indeed,  they  seem  to 
have  believed  it  possible  to  graft  almost  any  kind  of 
plants  together,  and  thereby  to  be  able  to  attain  to 
almost  any  desired  result. 

Grafting  as  practised  by  horticulturalists  consists  in 
removing  a  plant  or  a  part  of  a  plant,  which  is  known 
as  the  scion,  from  its  own  trunk,  stem,  or  root,  and 


FIQ.  149. — Different  modes  of  grafting:  I,  crown  grafting;  II,  splice  grafting; 
III,  bud  grafting;  W,  stock;  E,  scion.  (Slrasburger,  Noll,  Schenck,  and  Karsten.) 

transferring  it  to  another  trunk,  stem,  or  root  wThich  is 
known  as  the  stock, 

It  has  a  very  useful  function  in  that  it  enables  the 
operator  to  make  use  of  easily  cultivable  stocks  for  the 
purpose  of  supporting  difficultly  cultivable  scions.  Thus, 
many  of  the  luscious  fruits  are  with  great  difficulty 
reproduced  from  seeds  and  many  of  the  most  beautiful 


440  BIOLOGY:  GENERAL  AND  MEDICAL 

flowers,  being  hybrids  and  infertile,  cannot  be  raised 
from  seeds  and  so  would  be  lost  were  it  not  possible  to 
propagate  them  either  by  slipping  or  grafting.  The  graft- 
ing of  such  plants  also  removes  the  risk  of  sporting  and 
reversion  that  would  undoubtedly  occur  if  seeds  of  the 
fertile  kinds  were  alone  depended  upon  for  their  propa- 
gation. Slow-growing  fruit  trees  that  might  not  bear 
fruit  for  eight  or  ten  years  can  be  made  to  bear  in  one 
or  two  years  by  grafting  them  upon  already  well-grown 
trees  of  inferior  quality. 

The  stock  that  furnishes  the  roots  is  derived  from  one 
plant,  the  scion  that  will  bear  the  fruit  is  derived  from 
another  and  usually  superior  plant,  Will  the  sap  ascend- 
ing from  the  inferior  stock  into  the  superior  scion  effect 
any  change  in  it,  or  will  the  returning  sap  from  the  scion 
descending  into  the  stock  modify  it?  In  the  event  of  the 
scion's  being  but  one  of  many  branches  of  the  same  plant, 
will  the  products  of  the  scion  descending  into  the  stock 
and  then  returning  to  the  other  branches  modify  them? 

It  seems  difficult  to  get  at  the  exact  facts.  As  has 
been  said,  the  ancients  believed  in  these  modifications 
and  laid  great  stress  upon  them.  Some  modification 
would  be  consistent  with  what  has  been  found  in  certain 
cases  of  grafting  among  animals,  as  when  the  graft  of 
negro  skin  becomes  white  by  removal  of  its  pigment,  etc., 
not  with  others,  as  the  retention  of  their  relative  specific 
characteristics  by  the  anterior  and  posterior  halves  of  a 
frog  developing  from  a.  tadpole  composed  of  halves 
derived  from  different  individuals  of  different  species. 

The  subject  has  been  carefully  considered  by  Daniel, 
who  concludes  that,  "To  say  that  no  variation  takes 
place  in  the  graft  is  the  mistake  of  the  moderns;  to 
believe  that  variation  is  constant,  regular,  and  capable 
of  any  modification  is  the  error  of  the  ancients.  The 
truth  is  to  be  found  between  these  two  equally  exag- 
gerated opinions." 

As  the  result  of  his  survey  of  the  subject  and  of  his 
own  interesting  experiments,  Daniel  came  to  the  con- 


GRAFTING  441 

elusion  that  "the  graft  does  influence  the  general  nutri- 
tion of  the  plant,  and  that  its  influence  is  manifested: 

"1.  By  modifying  the  dimensions  of  the  vegetative  apparatus 
of  the  subject  and  of  the  graft. 

"2.  By  modifying  the  taste  and  the  size  of  the  edible  parts, 
their  chemical  composition,  and  the  time  of  their  appearance  upon 
the  plant. 

"3.  By  modifying  the  rapidity  with  which  the  reproductive 
organs  appear  upon  the  graft. 

"4.  By  modifying  the  relative  resistance  of  the  two  plants  to 
parasites  and  to  external  agents. 

"The  physiological  copartnership  seems  to  be  entered 
into  upon  restricted  lines.  The  general  structure  of  the 
scion  and  the  stock  remain  unchanged :  each  has  its  own 
forms  of  tissues,  its  own  mode  of  secondary  growth, 
its  own  formation  of  bark,  and  maintains  its  strong 
individuality." 

Strasburger  points  out  "that  the  scion  and  stock  do 
exert  some  influence  upon  one  another,  for  when  annual 
plants  are  grafted  upon  biennial  or  perennial  stocks, 
they  attain  an  extended  period  of  existence."  He 
also  adds  that  "in  special  cases  they  do  mutally. exert, 
morphologically,  a  modifying  effect  upon  each  other 
(graft  hybrids)." 

McCallum  gives  a  number  of  interesting  examples  of 
modifications  in  scion  and  stock  following  grafting. 
"  In  the  leaves  of  Epiphyllum  are  found  certain  albumen 
bodies  not  found  in  the  leaves  of  the  related  plant 
Peireskia.  Mitosch  grafted  Epiphyllum  scions  upon 
Perireskia  stocks,  and  in  the  leaves  which  subsequently 
developed  upon  the  latter  found  similar  bodies." 

The  most  interesting  graft-hybrid,  and  the  only  one  it 
seems  worth  while  to  mention,  is  the  Cytisus  adami, 
which  is  a  most  striking  example  of  what  may  happen 
when  grafting  is  successful.  The  Cytisus  vulgare  is  a 
large  tree  bearing  racemes  of  yellow  flowers;  Cytisus 
purpureus  a  shrub  of  small  size  bearing  racemes  of 
small  purple  flowers.  In  1826,  J.  L.  Adam  tried  the  ex- 
periment of  grafting  the  latter  upon  the  former  and 


442  BIOLOGY:  GENERAL  AND  MEDICAL 

produced  a  surprising  hybrid  which  grew  into  a  large 
tree  upon  which  appeared  large  numbers  of  the  usual 
yellow  racemes  of  Cytisus  vulgare,  large  numbers  of 
reddish  racemes  of  equal  size,  and,  what  was  most  pecu- 
liar, distributed  over  the  tree  like  gay  parasites,  there 
were  groups  of  small  boughs  upon  which  were  numbers 
of  the  small  purple  racemes  of  Cytisus  purpureus. 
Presumably  such  an  effect  could  only  be  brought  about 
through  a  partial  fusion  of  the  protoplasts  of  stock  and 
graft  in  the  callus  formed  during  the  healing  of  the 
graft  wound.  Interesting  graft-hybrids  have  also  been 
produced  by  Winkler. 


REFERENCES. 

THOMAS  H.  MORGAN:     "Regeneration,"  N.  Y.,  1901. 

C.   C.   GUTHRIE:     "  Heterotransplantation  of   Blood  Vessels  and 

Other  Studies."   Dissertation,  Chicago,  1907.    "Science," 

xxiv,  July,  1909. 
G.    BORN:      "Ueber     Verwachsungsversuche     mit    Amphibien- 

larven."    Archiv.  f .  Entwickelungsmechanik  der  Organis- 
.  men,  1897,  iv.,  349. 
ALEXIS  CARREL:     "Transplantation    in    Mass  of    the  Kidneys." 

Journal  of  Medical  Research,  1908,  x.,  98. 
ALEXIS    CARREL:      "Results   of   the   Transplantation   of    Blood 

Vessels,  Organs,  and  Limbs."     Jour.  Amer.  Med.  Asso., 

li.,  No.  20,  Nov.  14,  1908,  p.  1662. 
C.  C.  GUTHRIE:     "Survival  of  Engrafted  Tissues."     Jour.  Amer. 

Med.  Asso.,  March  12,  1910,  liv,  No.  11,  p.  831. 
C.  C.  GUTHRIE:     Survival  of    Engrafted    Tissues:    Ovaries    and 

Testicles."    Jour,  of  Experimental  Medicine,  1910,  xii., 

269. 
H.  E.  CRAMPTON:    "Annals  of  the  New  York  Academy  of  Science," 

1898,  xi.,  No.  11,  p.  219. 


CHAPTER  XVIII. 
SENESCENCE,  DECADENCE,  AND  DEATH. 

Ernest  Thompson  Seton  has  done  much,  through 
his  wild-animal  stories,  to  acquaint  his  readers  with  the 
tragic  circumstances  with  which  the  lives  of  the  wild 
creatures  usually  terminate,  and  has  thus  brought  them 
to  understand  the  "struggle  for  existence."  In  spite 
of  the  appalling  number  of  unfavorable  conditions  to  be 
overcome,  enemies  to  be  fought,  parasites  to  be  endured, 
infections  to  be  survived,  enough  living  things  man- 
age to  grow  old  to  show  us  that  for  each  kind  there 
seems  to  be  a  certain  age  limit  beyond  which  survival 
is  impossible  because  of  internal  changes  resulting  from 
the  inevitable  anatomico-physiological  wear  and  tear. 
These  changes  are  best  known  in  man  and  the  domestic 
animals,  for  among  the  wild  creatures  they  subject  the 
individual  to  insuperable  handicaps  in  the  struggle  for 
existence. 

We  are  accustomed  to  think  of  living  things  as  mortal, 
and  it  is  difficult  to  escape  this  conviction.  Living 
things  as  individuals  are  mortal,  but  the  germ-plasm 
is  immortal  and  continuous. 

Unicellular  organisms  whose  multiplication  takes 
place  by  fission,  and  whose  individuals  periodically 
rejuvenate  their  substance  by  conjugation,  escape  old 
age.  There  seems  to  be  no  reason  apart  from  accident 
why  any  of  them  should  die. 

The  same  obtains  among  such  multicellular  organisms 
as  multiply  by  gemmation.  It  is  the  substance  of  the 
parent  of  which  the  offspring  is  formed,  and  the  ancestral 
substance  is  present  in  every  individual.  The  condition 
is  not  materially  altered  when  the  sexual  mode  of  repro- 
443 


444  BIOLOGY:  GENERAL  AND  MEDICAL 

duction  is  reached,  for  the  gametes  derived  from  the 
parents  mingle  their  substance  in  the  zygocyte  or  fer- 
tilized ovum  and  form  the  starting  point  of  the  new 
generation  and  the  germ-plasm  universally  pervades 
the  species  and  is  handed  down  from  generation  to 
generation. 

The  soma-plasm  that  grows  about  the  germ-plasm  and 
subserves  the  purpose  of  transmitting  it,  grows  old  and 
dies,  but  before  that  time  the  germ-plasm  is  usually 
transmitted  to  a  new  generation  by  which  it  is  trans- 
mitted to  other  individuals,  and  so  on  forever. 

"The  germ-plasm  is  like  some  great  legacy:  the  trustees 
grow  old  and  die,  but  the  fund  goes  on  forever."  The 
individual  is  but  an  incident  in  the  life  of  the  germ-plasm. 

The  soma  develops  through  activities  contained  in 
the  germ,  all  its  conduct  is  predetermined  in  the  germ, 
the  method  by  which  the  germ-plasm  is  to  be  trans- 
mitted to  a  new  soma  is  predetermined  in  the  germ, 
and  the  time  of  the  decadence  of  the  custodial  soma  is 
predetermined  in  the  germ. 

The  more  complexly  differentiated  the  soma  becomes, 
the  more  difficult  it  is  to  sustain  and  the  privilege  of 
conjugation  by  which  rejuvenation  of  the  cells  seems 
to  be  effected  being  reserved  for  the  germ-plasm  alone, 
the  decadence  of  the  soma  becomes  inevitable. 

The  longevity  of  the  soma-plasm  is  extremely  variable 
and  the  laws  by  which  it  is  determined  are  unknown. 
Among  different  living  things,  great  differences  in  lon- 
gevity obtain.  Some  insects  live  but  days  or  even  hours; 
the  great  sequoia  trees  live  for  thousands  of  years. 

But  even  among  closely  related  living  things  there  are 
great  differences  in  longevity.  The  sequoia  trees  live 
for  thousands  of  years,  but  some  of  the  firs  become 
decadent  in  twenty  years.  Ravens,  crows,  and  parrots 
live  from  fifty  to  a  hundred  years,  but  most  other  birds 
only  one  or  two  decades. 

The  whole  subject  of  longevity  is  vague  and  few  prin- 
ciples regarding  it  can  be  formulated.  Those  creatures 


SENESCENCE,  DECADENCE,  AND  DEATH 


445 


seem  to  live  longest  that  are  longest  in  arriving  at  ma- 
turity, but  there  are  such  striking  exceptions  that  one 
hesitates  in  making  this  a  rule.  For  example,  certain 
ants  that  reach  maturity  in  a  few  weeks  are  known  to 
live  several  years,  while  a  species  of  Cicada  remains  a 
subterranean  nymph  for  seventeen  years  and  then 


F 


FIG.  150. — Hair  about  to  become  gray.    Cbromophages  transporting  the  pigment 
granules.     (Mttchnikoff.) 

emerges  from  the  ground  to  enjoy  but  a  few  days  of 
adult  life. 

Too  little  attention  has  been  paid  to  the  phenomena 
of  senescence  to  give  us  any  clear  understanding  of 
them.  We  are  even  uncertain  how  many  of  the 
changes  found  in  aged  human  beings  are  purely  senile 
and  not  the  results  of  antecedent  ailments.  If  we  view 
the  senile  state  from  the  point  of  view  of  wear  and  tear, 
we  are  not  infrequently  confronted  by  the  paradoxical 


446  BIOLOGY:  GENERAL  AND  MEDICAL 

discovery  of  an  extremely  aged  person  in  whose  dead 
body  the  supposedly  characteristic  changes  are  absent. 
Two  savants  have  devoted  particular  attention  to  the 
phenomena  of  senescence.  Charles  Sedgwick  Minot 
refers  all  of  the  senile  changes  to  cellular  transformations 
which  he  calls  "  cytomorphosis."  His  views  are  sum- 
marized in  the  following  four  laws: 

1.  Rejuvenation  depends  on  the  increase  of  the  nuclei. 

2.  Senescence  depends  on  the  increase  of  the  proto- 
plasm and  on  the  differentiation  of  the  cells. 

3.  The    rate   of   growth   depends   on   the   degree   of 
senescence. 

4.  Senescence  is  at  its  maximum  in  the  very  young 
stages,  and  the  rate  of  senescence  diminishes  with  age. 
According  to  Minot,  the  completion  of  embryonal  de- 
velopment begins  the  period  of  senescence   which  pro- 
gresses rapidly  as  the  individual  grows  into  an  adult, 
and  more  slowly  thereafter. 

Elie  Metchnikoff  refers  the  senile  changes  to  the  in- 
creasing activity  of  phagocytic  cells  by  which  the  less 
active  somatic  cells  are  slowly  destroyed. 

There  seems  to  be  truth  in  both  doctrines.  The 
more  completely  the  cells  are  differentiated  and  special- 
ized, the  less  independent  and  less  vital  they  become  and 
the  more  readily  they  fall  into  decay. 

Let  us  now  examine  the  human  body  to  determine 
what  may  be  regarded  as  the  usual  signs  of  old  age  and 
in  what  manner  they  contribute  to  final  dissolution 
and  death.  Be  it  understood,  however,  that  the  order 
in  which  the  recognized  conditions  are  considered  is  not 
by  any  means  the  necessary  order  of  their  occurrence. 
Indeed  it  seems  impossible  to  determine  the  exact  chro- 
nology of  the  changes  as  almost  any  of  the  important  dis- 
turbances may  establish  a  "vicious  circle"  by  which  it 
may  become  intensified,  as  well  as  other  retrogressive 
changes  inaugurated. 

With  increasing  age  we  find  atrophic  changes  in  all  the 
organs  and  tissues  of  the  body.  This  atrophy  is  gener- 


SENESCENCE,  DECADENCE,  AND  DEATH     447 

ally  characterized  by  loss  of  the  cellular  tissues  and 
increase  of  the  fibrillar  tissues  and  is  accompanied  by 
diminished  functional  activity  of  all  the  parts  involved. 

The  heart  is  usually  small,  its  muscle  brown  and  tough, 
and  the  subepicardial  tissue  either  abnormally  fatty  or 
quite  devoid  of  adipose  tissue.  The  muscle  cells  are 
usually  excessively  pigmented. 

The  lungs  usually  show  more  or  less  widespread  emphy- 
sema. This  probably  depends  upon  the  loss  of  the 
elastic  tissue  and  the  permanent  overdistention  of  the 
air  vesicles  in  consequence.  The  changes  usually  occur 
first  at  the  sharp  anterior  edges  and  apices  of  the  lungs, 
but  may  be  universal. 

The  stomach  may  be  quite  small,  the  glandular  tubules 
diminished  in  number,  the  muscle  thinned,  and  the 
fibrillar  tissue  increased.  The  loss  of  the  glandular 
tissue  impoverishes  the  enzymic  content  of  the  gastric 
juice  as  well  as  diminishes  its  quantity;  the  disturbance 
of  the  muscular  tissue  weakens  the  peristaltic  action,  and 
not  infrequently  the  muscle  yields  to  the  distention  of 
gaseous  contents,  when  fermentative  changes  occur 
through  deficiency  of  the  gastric  juice. 

The  intestinal  walls  are  thinned,  and  many  of  the  rugae 
of  the  jejunum  and  upper  ileum  obliterated.  The  colon 
may  be  contracted  and  thinned  or  may  be  dilated  and 
still  more  thinned. 

The  liver  usually  shows  brown  atrophy.  It  is  small 
in  size,  pigmented,  and  indurated.  The  quantity  of  bile 
is  diminished,  and  the  urea  forming  and  glycolytic  func- 
tions disturbed. 

The  pancreas  usually  shows  more  or  less  atrophy  of  the 
secreting  structure,  increase  of  the  interstitial  tissue,  and 
atrophy  of  the  bodies  of  Langerhans.  These  bodies  are 
not  infrequently  the  seat  of  hyaline  degeneration. 

The  kidneys  show  more  or  less  atrophy  of  the  paren- 
chyma and  increase  of  the  interstitial  tissue,  and  in 
addition  commonly  show  localized  atrophic  areas  ref- 
erable to  changes  in  the  blood  vessels.  By  comparing 


448  BIOLOGY:  GENERAL  AND  MEDICAL 

a  large  number  of  kidneys  of  various  ages,  from  one  year 
or  less  up  to  ninety-nine  years,  Walsh  has  found  that 
the  proportion  of  connective  tissue  between  the  ducts 
at  the  apical  portion  of  the  pyramids  regularly  increases 
with  age. 

The  muscular  tissues  are  everywhere  atrophic  and 
show  a  distinct  increase  of  fibrillar  tissue,  which  accounts 
for  the  general  muscular  weakness  of  the  aged.  As 
these  changes  are  not  confined  to  the  voluntary  muscles, 
but  also  affect  the  cardiac  and  involuntary  muscles,  they 
explain  the  general  cardiac  weakness  and  the  inactivity 
of  the  alimentary  canal.  They  also  explain  the  tough- 
ness of  the  flesh  of  old  animals.  The  loss  of  muscular 
tone  is  also  responsible  for  the  relaxations  of  the  dorsal 
muscles  by  which  the  altered  carriage  of  the  aged  is 
partly  brought  about,  and  the  loss  of  tone  in  the  abdomi- 
nal muscles  explains  the  frequency  with  which  umbilical 
and  other  herniae  occur  or  enlarge  in  the  aged. 

The  bones  show  pronounced  atrophic  changes.  The 
anatomical  landmarks  indicative  of  muscular  attach- 
ments, etc.,  well  marked  in  youth,  gradually  become 
obliterated  and  the  surfaces  smoothed  over.  The  cranial 
sutures  become  obliterated  and  the  bones  united.  The 
thinner  processes  of  the  bones  are  gradually  absorbed. 
This  is  perhaps  best  exemplified  by  the  alveolar  proc- 
esses of  the  maxillary  bones  whose  atrophy,  accompanied 
by  the  recession  of  the  gingival  tissues  from  the  teeth,  is 
followed  by  looseness  of  these  organs,  which  fall  out, 
even  if  they  have  not  already  undergone  decay.  The 
loss  of  the  teeth  and  the  absorption  of  the  alveolar  proc- 
esses cause  an  approximation  of  the  nose  and  chin, 
characterized  as  the  "nut-cracker  face."  In  atrophy 
of  the  bones  the  too  busy  osteoclasts  misapply  their  en- 
ergy, so  that  the  compact  layers  become  more  and  more 
dense  and  brittle,  leaving  the  cancellous  tissue  insuf- 
ficiently supported.  Certain  portions  of  the  skeleton — 
notably  the  necks  of  the  femurs — also  tend  to  bend. 
There  is  also  a  disposition  for  bone  to  form  in  unusual 


SENESCENCE,   DECADENCE,    AND   DEATH  449 

situations,  such  as  between  the  bodies  of  the  vertebrae 
and  about  the  intervertebral  discs,  so  that  individual 
vertebrae  become  welded  together  and  immovably 
fixed.  If  the  muscles  of  the  erector  spinae  group  have 
permitted  the  body  to  drop  forward,  the  spine  may 
become  hopelessly  fixed  in  this  curved  position.  It  is 
partly  through  such  bony  changes  that  the  height  of 
the  aged  person  becomes  considerably  reduced. 

The  skin  and  its  appendages  show  well-marked  atrophic 
changes.  The  first  of  which  may  be  whitening  of  the 
hair.  This  has  been  found  by  Metchnikoff  to  depend 
upon  the  absorption  of  the  pigment  from  the  cells  of  the 
medulla  of  the  hair  by  phagocytic  cells — pigmentophages 
— through  whose  activity  it  is  first  transferred  to  the 
bulb  of  the  hair,  and  subsequently  removed  altogether' 
With  or  without  the  loss  of  the  color,  the  hair  follicles 
may  atrophy  and  baldness  occur.  Though  such  changes 
occur  upon  the  scalp  and  beard  and  about  the  pubes 
and  axilla,  the  hairs  of  the  beard  and  eyebrows  are  apt 
to  become  coarse  and  bushy,  and  coarse  hairs  frequently 
appear  upon  the  ears  and  elsewhere.  The  skin  itself 
becomes  thinned,  shining,  more  or  less  transparent, 
and  marked  with  brownish  discolorations.  The  sense  of 
touch  is  impaired,  so  that  it  is  probable  that  the  sensory 
end  organs  of  the  nerves  also  atrophy  and  disappear  in 
part.  The  conjunctiva  loses  in  whiteness  and  may  show 
occasional  yellowish  fatty  deposits.  About  the  corneal 
margins  such  a  fatty  deposit  receives  the  name  arcus 
senilis. 

The  sexual  organs  participate  in  the  senile  changes, 
those  of  women  more  early  than  those  of  men.  With 
women  the  first  change  is  physiological  and  is  shown  by 
the  cessation  of  menstruation — menopause.  This  is 
soon  followed  by  atrophy  and  cirrhosis  of  the  ovaries, 
more  or  less  atrophy  of  the  uterus,  and  involution  or 
atrophy  of  the  mammary  glands  in  which  the  glandular 
tissue  is  in  part  replaced  by  adipose  tissue. 

In  men  the  atrophic  changes  of  the  sexual  organs  is 
postponed  for  a  considerable  time,  so  that  the  sexual 

29 


450  BIOLOGY:  GENERAL  AND  MEDICAL 

life  of  a  man  is  considerably  longer  than  that  of  a  woman. 
Eventually,  however,  the  testes  show  atrophy  and  pig- 
mentation of  the  cells  of  the  seminiferous  tubules  and 
the  formation  of  spermatozoa  almost  ceases.  The  pros- 
tate gland  sometimes  enlarges  in  old  men,  but  in  not  a 
few  aged  men  follows  the  usual  rule  and  atrophies. 

The  blood-vascular  system  undergoes  a  general  fibrosis, 
chiefly  characterized  by  loss  of  muscular  and  elastic 
tissue.  The  endarterium  is  prone  to  suffer  from  pro- 
liferation of  the  subendothelial  tissues  and  more  or  less 
obstruction.  These  changes  may  be  accompanied  by 
more  or  less  saponaceous  change  followed  by  calcifica- 
tion. If  calcification  chiefly  localizes  in  the  middle 
coat,  the  vessels  may  be  transformed  to  mineralized 
hollow  cylinders — "pipe-stem  arteries";  when  it  local- 
izes in  the  intima,  calcareous  plates  appear  in  the  vessel 
walls.  Fibroid  changes  cause  the  vessels  to  lose  their 
elasticity,  increase  the  blood  pressure,  throw  an  addi- 
tional strain  upon  the  heart,  and  further  damage  some  of 
the  viscera,  especially  the  kidneys.  If  the  changes  are 
internal  and  obstructive,  they  eventuate  in  atrophy  of 
the  tissue  to  which  the  particular  vessel  distributes. 
Calcareous  vessels  become  brittle- and  liable  to  fracture 
with  resulting  hemorrhage.  Apoplexy,  that  so  fre- 
quently carries  off  the  aged,  commonly  results  from 
the  rupture  of  such  vessels  in  the  brain  and  the  destruc- 
tion and  compression  of  the  cerebral  substance  by  the 
escaping  blood.  When  the  peripheral  arteries  are  the 
seat  of  sclerosis  and  calcification  and  are  consequently 
obstructed,  the  limbs  are  sometimes  insufficiently 
nourished  and  gangrene  of  the  extremities — usually 
of  the  toes  and  feet — may  supervene. 

The  nervous  system  suffers  considerably.  There  can 
be  no  doubt  but  that  all  of  the  organs  of  special  sense 
are  more  or  less  embraced  in  the  general  atrophic  con- 
ditions for  the  acuity  of  all  these  senses  is  usually  ob- 
tunded.  The  vision  becomes  more  and  more  dimmed, 
the  hearing  is  dulled,  the  senses  of  taste  and  smell  are 
enfeebled.  But  the  central  nervous  system  also  suffers. 


SENESCENCE,    DECADENCE,    AND    DEATH 


451 


The  brain  not  infrequently  suffers  from  more  or  less 
well-defined  areas  of  arteriosclerotic  atrophy  and  soften- 
ing. Metchnikoff  has  observed  that  in  old  parrots  the 
nerve  cells  become  surrounded  by  phagocytic  cells — neu- 
rophages — that  gradually  encroach  upon  and  destroy 
them.  Such  destruction  of  the  nervous  tissues  inevit- 
ably results  in  changes  in  the  psychic  condition  of  the 
individual. 


FIG.  151. — Nerve  cells  surrounded  by  neurophagea — phagocytic  cells — by 
which  they  are  gradually  destroyed.  This  form  of  phagocytic  activity  only 
occurs  during  old  age.  (Metchnikoff.) 

These  anatomical  and  histological  changes  amply 
explain  the  defective  physiology  of  senility.  The  cardiac 
weakness  and  defective  vessels  are  inadequate  to  pro- 
vide that  free  circulation  by  which  alone  the  integrity 
of  the  tissues  can  be  maintained,  and  there  is  a  tendency 
for  widespread  calcification  to  make  its  appearance. 


452  BIOLOGY:  GENERAL  AND  MEDICAL 

It  is,  therefore,  found  in  the  costal  and  other  cartilages, 
in  the  blood-vessel  walls,  and  sometimes  in  other  tissues. 

As  the  calcareous  changes  come  on,  the  dentine  of  the 
teeth  becomes  denser  and  the  color  of  the  teeth  loses  in 
whiteness  and  gradually  increases  in  yellowness. 

The  digestive  organs  being  altered,  the  digestive  func- 
tions are  inadequate  or  defective.  The  eliminative 
organs  are  not  only  themselves  defective  in  structure, 
but  are  embarrassed  with  the  imperfectly  transformed 
metabolic  products  by  which  further  changes  in  their 
substance  are  induced.  The  general  oxidation  processes 
are  disturbed;  it  is  difficult  to  maintain  the  tempera- 
ture, and  fatigue  and  exhaustion  supervene  upon  slight 
effort.  In  some  cases  -a  tendency  to  obesity  presents 
itself. . 

The  psychic  conditions  are  adequately  explained  by 
the  destruction  of  the  nervous  tissue  through  arterio- 
sclerotic  atrophy  and  softening  and  by  the  phagocytic 
destruction  of  the  nerve  cells.  It  is  more  difficult  to 
explain  the  peculiar  character  of  the  decadent  men- 
tality, for  those  familiar  with  old  people  well  know  how 
acute  the  aroused  mind  may  be  in  contrast  with  its 
usual  apathy,  and  must  have  observed  how  much  more 
vivid  are  early  impressions  than  recent  ones. 

It  naturally  follows  that  such  enfeeblement  of  the 
vital  powers  gradually  causes  life  to  hang  upon  a  very 
slender  thread,  so  that  infections  that  might  easily 
have  been  overcome  in  the  vigor  of  youth  are  quickly 
fatal — pneumonia  being  one  of  the  most  common  causes 
of  death  among  the  aged.  A  severe  mental  or  physical 
shock  disturbs  the  delicate  equilibrium  and  the  weakened 
heart  may  stop.  Indeed,  in  cases  in  which  the  process  of 
decadence  has  been  slow,  and  the  anatomical  and  physio- 
logical disturbances  fairly  uniform,  no  other  cause  for 
death  can  be  assigned  than  the  gradual  exhaustion  of 
the  vital  powers. 

In  complexly  organized  beings  it  becomes  necessary 
to  distinguish  between  somatic  and  molecular  death. 


SENESCENCE,  DECADENCE,  AND  DEATH     453 

Somatic  death  refers  to  the  death  of  the  individual, 
molecular  death  to  the  death  of  his  component  cells. 
Among  the  higher  vertebrates  it  is  a  distinction  easy 
to  make,  but  as  the  scale  of  life  is  descended  it  becomes 
more  and  more  difficult.  The  life  of  the  higher  animal 
rests  upon  a  tripod  consisting  of  the  three  fundamental 
functions — circulation,  respiration,  and  innervation. 
These  are  indispensable  functions,  the  suspension  of  any 
one  of  which  causes  somatic  death  in  a  few  moments. 
We  do  not  say,  however,  that  the  individual  is  dead  until 
all  three  functions  have  ceased.  There  are  other  func- 
tions whose  suspension  is  equally  fatal,  though  the  end 
is  approached  more  slowly.  Thus  should  the  kidneys 
be  removed,  their  arteries  ligated,  or  their  function 
otherwise  set  aside,  death  is  inevitable,  nothing  can 
possibly  save  life,  but  the  end  is  approached  gradually 
and  comes  after  much  suffering  through  the  final  inter- 
ruption of  either  the  circulation,  the  innervation,  or  the 
respiration. 

The  tissues  and  their  component  cells  continue  to  live 
on  for  some  time  after  somatic  death  has  occurred,  the 
actual  duration  of  life  depending  upon  their  individual 
vitality  and  the  quantity  of  absorbable  and  still  utilizable 
nutrient  juices  available. 

There  is  great  dissimilarity  among  the  different  tissues 
in  this  particular.  The  nervous  tissues  seem  to  die 
quickly;  the  muscular  tissues  preserve  their  contractile 
power  for  some  time.  The  epithelial  cells  of  the  skin 
and  of  the  hair  follicles  seem  to  live  for  some  days,  so 
that  it  is  quite  possible  for  the  hair  to  grow  a  few  milli- 
metres after  death — perhaps  in  some  cases  even  more. 
Soon  after  death  certain  chemical  changes  set  in  and 
poison  those  cells  that  might  otherwise  survive  longer. 
Such  are  responsible  for  the  rigor  mortis  or  stiffness  of 
the  muscles  brought  about  by  coagulation  of  the  myosin. 

As  we  descend  the  scale  of  animate  life,  we  find  a 
general  tendency  toward  the  prolongation  of  molecular 
life  after  somatic  death.  This  can  probably  be  explained 


454  BIOLOGY:  GENER\L  AND  MEDICAL 

by  the  differing  chemical  conditions  in  the  bodies  of  the 
lower  forms. 

Thus  in  the  familiar  superstition  that  a  dead  snake 
moves  its  tail  "till  sundown,"  and  in  the  tendency  for 
the  amputated  head  of  a  snapping  turtle  to  bite  and 
hold  fast  to  a  stick,  or  in  the  tendency  of  a  decapitated 
rattlesnake  to  coil  and  strike,  we  see  examples  of  pro- 
longed molecular  or  cellular  animation  after  somatic 
death  has  occurred. 

If  we  descend  still  lower,  it  becomes  impossible  to 
make  any  clear  differentiation  between  the  somatic 
and  molecular  death  except  by  experiment.  Thus, 
when  an  earth-worm  is  chopped  to  pieces,  all  the  frag- 
ments live  on  for  a  considerable  time — many  days — and 
it  is  only  experience  with  the  regenerative  powers  of  this 
animal  that  enables  one  to  predict  which  fragments 
may,  and  which  may  not  live  and  form  new  worms. 

Still  more  interesting  is  the  condition  found  in  the 
hydra.  The  animal  is  cut  to  pieces,  each  piece  draws 
itself  together,  becomes  inactive  and  apparently  dead, 
but  after  a  time  quite  small  pieces  may  rearrange  the 
substance,  fit  themselves  for  further  growth  and  may 
recover. 

The  relation  of  molecular  to  somatic  life  and  death 
depends  upon  the  degree  of  specialization  attained  by 
the  cells  and  their  ability  to  maintain  more  or  less 
independence  under  unfavorable  condition. 

This  is  well  exemplified  by  the  behavior  of  plants 
among  whom  somatic  life  is  very  slightly  differentiated 
from  molecular  life  because  of  the  absence  of  such  special- 
ized organs  as  those  controlling  the  functions  of  circula- 
tion, innervation,  respiration,  and  digestion  in  animals. 
Partly  for  this  reason  and  partly  because  of  the  general 
diffusion  of  reproductive  energy  among  the  vegetative 
cells,  portions  of  many  plants  cut  off  from  the  parent  and 
placed  under  appropriate  conditions  live  on,  grow  well, 
and  soon  renew  the  whole  plant. 

The  somatic  life  of  each  individual  eventually  comes 


SENESCENCE,  DECADENCE,  AND  DEATH     455 

to  an  end,  but  the  life  of  the  germ-plasm  persists  in  its 
descendants  in  a  succession  of  ever-changing  forms  for 
which  one  can  see  no  end  so  long  as  the  physical  condi- 
tion of  our  planet  continues  to  afford  such  conditions  of 
temperature  and  moisture  as  are  compatible  with  life 
as  we  know  it. 

The  death  of  the  soma  and  the  succeeding  retrogressive 
and  analytic  changes  it  undergoes  must  not  be  inter- 
preted as  loss.  The  greater  part  of  the  surface  of  the 
earth  is  covered  by  a  layer  of  matter  composed  of  the 
products  of  organic  decomposition  which  is  continually 
utilized  by  new  living  things.  As  one  living  organism 
dies  and  disintegrates,  others  of  different  kind  arise  from 
its  remains,  so  that  the  organic  compounds  are  per- 
petually undergoing  cyclical  integration  and  disinte- 
gration, in  which  available  material  is  worked  over  and 
over  again  with  ever  new  results  by  new  organisms  aris- 
ing through  the  energy  of  the  immortal  germ-plasm. 

REFERENCES. 

CHARLES  SEDGWICK  MINOT:  "The  Problem  of  Age,  Growth,  and 
Death:  A  Study  of  Cytomorphosis  Based  on  Lectures 
at  the  Lowell  Institute,"  N.  Y.,  1908. 

ELIE  METCHNIKOFF:  "The  Nature  of  Man,"  translated  by  I.  C. 
Mitchell,  N.  Y.,  1903.  "Etudes  Biologiques  sur  la 
Veilleuses,"  Annales  d.  1'Inst.  Pasteur,  xv.,  xvi.,  1901- 
1902. 


CHAPTER   XIX. 
THE  CULTIVATION  OF  TISSUE  IN  VITRO. 

IT  has  been  shown  that  the  death  of  the  individual 
(somatic  death)  among  higher  animals  is  soon  followed  by 
the  death  of  its  parts  (molecular  death).  But  when  one 
considers  why,  it  appears  that  the  two  stand  in  relation- 
ship to  one  another  only  in  the  sense  that  somatic  life  is 
the  great  source  of  provision  and  regulation  by  which  the 
cells  are  fed,  exercised,  and  relieved  of  their  waste  prod- 
ucts. Is  it  because  of  the  loss  of  oxygen  and  nourishment 
and  the  removal  of  waste  by  the  once  circulating  and  now 
stagnating  juices  that  the  cells  die?  If  some  artificial 
means  could  be  provided  for  maintaining  the  appropriate 
temperature,  admitting  the  necessary  oxygen,  supplying 
the  necessary  foods,  and  washing  away  waste  products, 
could  tissues  be  kept  alive  indefinitely? 

If  life  is,  as  there  is  every  reason  to  suppose,  a  physico- 
chemical  manifestation,  then,  given  appropriate  condi- 
tions, it  should  continue  under  unnatural  conditions,  and 
under  inappropriate  conditions  it  ought  to  cease  under 
natural  conditions,  which  is,  indeed,  the  case. 

To  know  that  a  tissue  kept  under  artificial  conditions 
is  living,  the  same  criteria  must  be  applied  as  are  used  to 
determine  that  any  supposed  living  organism  is  living. 
It  must  show  signs  of  life  by  movement,  by  multiplica- 
tion, by  metabolism,  etc.  Not  all  of  the  tissues  of  the 
body  of  a  highly  organized  animal  are  able  to  show  the 
usual  signs  of  life  manifested,  for  example,  by  the  protozoa. 
Many  of  them  are  so  highly  specialized  and  differentiated 
as  to  do  nothing  visible,  and  nothing  by  themselves,  as 
in  the  case  of  a  nerve  cell  of  a  sensory  ganglion.  Many 
of  them  belong  to  tissues  whose  reproductive  power  was 
456 


THE   CULTIVATION   OF  TISSUE   IN   VITRO  457 

lost  in  differentiation,  as  nerve  and  muscle  cells.  Such 
cells  or  tissues  containing  such  cells  would,  therefore,  be 
manifestly  ill  adapted  to  such  experiments  as  are  under 
consideration,  except  during  the  period  of  embryonal 
development,  when  they  multiply  actively  like  other  tissue 
elements  of  less  specialization. 

From  these  and  similar  conditions  that  may  suggest 
themselves  it  becomes  evident  that  to  test  the  viability  of 
fragments  of  tissue  independently  of  the  body  as  a  whole, 
choice  should  be  made  of :  1,  embryonal  tissues  whose  chief 
activity  is  vegetative  and  reproductive;  2,  adult  tissues  of 
low  specialization  whose  cells  easily  multiply  and  are 
known  to  endure  adverse  conditions  easily;  3,  tissues  of 
tumors,  whose  tissues  have  an  abnormal  tendency  to 
multiply. 

It  is  difficult  to  find  when,  how,  and  by  whom  the 
earliest  experiments  of  this  kind  were  made,  but  probably 
few  attempts  were  made  before  1892  when  Leo  Loeb, 
interested  in  the  study  of  cancer,  was  led  to  try  whether 
it  was  possible  to  make  skin  grow  under  artificial  con- 
ditions. He  used  for  the  experiment  bits  of  the  skin  of 
embryo  guinea-pigs  which  he  embedded  in  agar-agar  and 
then  inserted  in  the  tissues  of  adult  guinea-pigs.  The 
agar-agar  kept  them  separate  from  the  tissues  of  the  host 
animal  and  defended  them  from  the  attacks  of  phagocytes, 
etc.,  while  they  were  supplied  with  nourishment  by  the 
IjTiiph  of  the  host.  Under  such  conditions  the  tissues 
survived  for  a  time  and  appeared  to  grow,  as  mitotic 
changes  and  wandering  of  the  cells  were  observed.  This, 
however,  was  not  an  experiment  in  vitro,  and  more  nearly 
resembled  grafting. 

The  chief  researches  on  the  subject  took  place  between 
1907  and  1912.  They  were  begun  by  R.  G.  Harrison  and 
later  continued  by  Burrows,  Carrel  and  Burrows,  Lambert 
and  Hanes,  Ruth,  Jolly,  Murphy  and  Rous,  Carrel  and 
Ingebrigsten,  Lewis,  Foot,  Eveling,  Hyami  and  Tanaka, 
and  others. 

Harrison's  researches  paved  the  way  for  the  subsequent 


458  BIOLOGY:  GENERAL  AND  MEDICAL 

experiments,  most  of  which  were  performed  by  modifica- 
tions of  his  technic.  He  began  by  immersing  embryonal 
frog  tissues  in  coagulable  frog's  lymph,  taken  from  the 
lymph-sac  of  an  adult  frog.  His  studies  of  nerve  cells 
under  these  conditions  were  a  brilliant  success,  for  he 
found  not  only  that  it  was  possible  to  keep  the  cells  alive 
and  have  them  multiply,  but  he  was  able  to  see  how  the 
cells  grew  and  developed  and  to  demonstrate  that  the 
neuraxis  is  really  an  outgrowth  of  a  central  neuron. 

Harrison  and  Burrows  improved  the  technic  by  using 
plasma  of  the  blood  instead  of  lymph,  and  in  it  were  able 
to  cultivate  tissues  of  an  embryo  chick.  The  plasma  was 
dropped  upon  a  cover  glass,  the  bit  of  tissue  placed  in  the 
center  of  it,  and  the  whole  inverted  in  a  hollow-ground 
slide  and  kept  in  a  thermostat.  By  this  technic  they 
cultivated  bits  of  the  central  nervous  system,  of  the  heart, 
and  of  the  mesenchymal  tissues. 

Carrel  and  Burrows  took  up  the  pursuit  of  the  subject 
systematically  and  showed  that  by  this  method  and 
slight  modifications  of  it  it  was  possible  to  cultivate  al- 
most all  of  the  adult  tissues  of  the  dog,  cat,  chicken,  rat, 
and  guinea-pig  in  vitro.  According  to  their  nature  the 
tissues  generated  connective  tissue  or  epithelial  cells 
which  grew  in  the  plasmatic  medium  either  in  continuous 
layers  or  in  radiating  chains.  They  were  able  to  observe 
direct  division  of  the  nuclei  during  the  life  of  the  cells, 
and  the  occurrence  of  many  karyokinetic  figures  in  the 
fixed  and  stained  cultures.  The  usual  duration  of  the 
life  of  a  culture  was  about  two  or  three  weeks,  but  by 
transplantation  they  were  able  to  keep  the  cells  alive  and 
growing  for  a  much  longer  time. 

Rous,  using  the  same  methods,  was  able  to  keep  tissue 
from  a  tumor  (sarcoma  of  a  chicken)  growing  in  the  same 
way,  and  Lambert  and  Hanes  did  the  same  with  the 
tissue  of  a  mouse  tumor. 

Ruth  adapted  the  method  to  the  study  of  the  process 
of  healing,  Carrel  and  Burrows  adapted  it  to  the  investi- 
gation of  the  effects  of  internal  secretions  and  other  sub- 


THE    CULTIVATION   OF  TISSUE   IN   VITRO  459 

stances  upon  growth,  and  Ruth  applied  it  in  the  investiga- 
tion of  the  problems  of  immunity  in  cancer. 

Thus  a  considerable  interest  in  the  subject  was  aroused 
and  it  continued  to  be  investigated  with  enthusiasm,  and 
surprising  facts  were  ascertained,  as,  for  example,  that 
tissues  will  grow  in  artificially  prepared  media — Lock's 
solution — and  sometimes  in  heterologous  plasma.  Also 
that  tissues  kept  in  cold  storage  may  be  made  to  grow 
if  they  are  sterile  as  regards  bacteria  when  planted  in 
appropriate  media. 

Lambert  studied  the  formation  of  giant  cells  in  growing 
tissue,  Carrel  and  Ingebrigsten  studied  antibody  forma- 
tion in  tissue  growing  outside  of  the  body,  and  Carrel 
finally  announced  that  it  was  possible  to  keep  tissue  grow- 
ing indefinitely  outside  of  the  body  by  proper  transplanta- 
tion and  treatment.  This  led  to  one  of  the  most  interest- 
ing observations  in  the  whole  series,  and  confirmed  the 
opinion  of  physiologists  that  the  cardiac  muscle  contracts 
through  energies  intrinsic  in  it,  and  not  only  upon  stimula- 
tion by  nervous  energy  like  the  voluntary  muscles.  Frag- 
ments of  heart  muscle  from  an  embryo  chick  were  kept 
growing  in  vitro.  The  cells  spread  out  radially  forming  a 
thin  layer  that  could  be  easily  observed  under  the  micro- 
scope. At  the  end  of  sixty  days  they  were  observed  to 
begin  rhythmical  pulsation. 

It  will  be  interesting  to  see  whether  Carrel's  conclusions 
drawn  from  his  own  and  Lewis'  experiments  will  ever  be 
justified,  but  his  final  conclusion  of  the  whole  matter 
seems  to  be  that  it  will  some  day  be  possible  to  make  use 
of  cold  storage  for  the  purpose  of  keeping  alive  useful 
tissues  removed  by  the  surgeon,  so  that  they  can  subse- 
quently be  used  for  the  repair  of  injured  parts  and  tissues 
by  grafting. 

REFERENCES. 

LEO  LOEB:  "Ueber  die  Entstehung  von  Bindegewebe,  Leukocyten, 
und  rothen  Blutkorperchen  aus  Epithel  und  iiber  eine 
methode  isolirte  Gewebstheile  zu  Zuchten,"  Archiv.  f. 
Entwichelungsmechanik  d.  Organ.,  1902,  xiii,  487. 


460  BIOLOGY:  GENERAL  AND  MEDICAL 

R.  G.  HARRISON:    Proc.  Soc.  Exp.  Biol.  and  Med.,  1907,  iv,  140. 

Anatomical  Record,  1908,  ii,  385.     Jour.  Exp.  Zool.,  1910. 

ix,  787. 
CARREL  and  BURROWS:    Jour.  Amer.  Med.  Assoc.,  1910,  Iv,  1379, 

1559;  1911,  Ivi,  32.    Jour.  Exp.  Med.,  1910,  xii,  696;  1911, 

xiii,  390,  416,  562,  571;  xiv,  244. 
RUTH:  Jour.  Exp.  Med.,  1911,  xiii,  422,  559. 
LAMBERT  and  HANES:    Jour.  Exp.  Med.,  1911,  xiii,  495,  505;  xiv, 

129,  453. 

LEWIS:  Bull.  Johns  Hopkins  Hospital,  1911,  xxii,  126. 
MURPHY  and  Rous:  Jour.  Exp.  Med.,  1912,  xv,  119. 
CARREL  and  INGEBRIGSTEN:   Jour.  Exp.  Med.,  1912,  xv,  287. 
CARREL:      Jour.   Exp.    Med.,    1912,   xv,   516.     Jour.   Amer.  Med. 

Assoc.,  Ivii,  1911,  p.  1611;  1912,  lix,  2105;  1912,  lix,  523. 


INDEX 


ACANTHOCEPHALA,  340 

Acarus  scabiei,  351 

Accessory  idioplasm,  263 

Accidental  polyembryony,  218 

Acquired  immunity,  373 

Actinozoa,  339 

Adami's  theory  of  heredity,  274 

Aerobic  bacteria,  52 

Afferent  neuraxes,  152 

African  lethargy,  361 

Agassiz  on  classification,  308 

Aggressins,  364,  367 

Alexine,  387 

Allelomorphs,  269 

Allergia,  377 

Allman  on  geotropism,  66 

Alternation  of  generation,  197 

Altmann,  95 

Amboceptor,  379 

Amnion,  226 

Amoeba  and  electric  currents,  61 

conductivity  in,  69 

food  reactions  of,  49 

mechanical  stimulation,  44 

movement  in,  74 

structure  of,  110 
Amoeboid  movement,  74 
Amphiaster,  106 
Amphimixis,  116,  193 
Amphipyrenin,  98 
Anabolism,  83 
Anaphase,  105 
Anaphylaxis,  377 
Anaximander,  21 
Anchylostoma,  354 

duodenale,  340,  354 
Ancistrum,  338 
Anesthetics,  37 
Angiostomum,  209 


Animal  kingdom,  relations  of  di- 
visions of,  305 

Animalculists,  212 

Animals,  classification  of,  312 
conductivity  in,  70 
geotropism  in,  66 
reproduction  in,  200 

Annulata,  341 

Anopheles,  358 

Anophlophyra,  338 

Antheridium,  190,  199 

Anthers,  199 

Anthrax,  vaccine  for,  395 

Antibodies,  317,  376 

Antigen,  317,  376 

Antitoxins,  317 
therapeutic,  378 

Apanteles  congregatus,  346 

Arachnida,  347 

Archegonia,  199 

Archenteron,  222 

Archiblast,  221 

Archicele,  221 

Arcus  senilis,  449 

Aristole,  282,  301 

Arnold,  107 

Arthropoda,  341 

Ascaris  lumbricoides,  340,  354 

Ascogonium,  187 

Ascus,  188 

Asexual  reproduction,  181 

Asphyxia,  54 

Associative  memory,  172 

Atrophic  changes  of  age,  446 

Auricles  of  heart,  formation  of, 
139 

Autogenetic  cells,  259 

Automatic  action,  178 

Automatism,  178 


462 


INDEX 


Axolotl,  203 
Azygospores,  193 


BACILLUS,  colon,  334 

of  tetanus,  333 

toxic  product  of,  366 

of  tuberculosis,  334 
Bacteria,  aerobic,  52 

and  infection,  364 

and  oxygen,  52 

and  phagocytes,  386 

avenues  of  entrance,  366 

discovery,  of  23 

effect  of  temperature  on,  38 

life  of,  366 

light  and,  55 

mutualism  of,  330 

nitrogen-fixing,  330 

number  of,  366 

sitotropic  reactions  of,  49 
Balantidium  coli,  338 
Barry,  214 

Bastard  toadflax,  337 
Bateson   on   sex   determination, 

247 

Beck,  87 
Bed-bugs,  332 
Behavior,  169,  172 
Bilharzia,  339 

Binomial  nomenclature,  310 
Bioblasts,  95 
Biogenetic  law,  235 
Bjogenic  infusions,  25 
Biophors  of  Weismann,  260 
Bitter  root  fever,  350 
Blastocele,  221 
Blastoderm,  221 
Blastodermic  vesicle,  227 
Blastogenic  idioplasm,  263 
Blastomeres,  220 
Blastopore,  222 
Blastula,  197,  221 
Blood,  development  of,  134 

relationship,  317 

transfusion,  groups  for,  324 
Blood-cells,  95 

Blood-vessels,    regeneration    of, 
412 

replacement  of,  432 

suture  of,  420 
Bluraenbach,  240 


Bones,  atrophic  changes  in,  448 

regeneration  of,  413 
Bonnet  on  regeneration,  406 
Bordet  on  blood,  318 
Born  on  grafting  in  tadpoles,  435 
Botfly,  344 
Boveri,  209 

Box-within-box  doctrine,  213 
Brain,  development  of,  155 

importance  of,  165-168 
Branchiae,  144 
Breeds     of     domestic     animals, 

Darwin  on,  290 
Brem,  323 

Brooks'  theory  of  inheritance,  257 
Budding,  181 
Buffon  on  evolution,  283 
Bugs,  342 
Burrows  on  growth  of  tissue  in 

vitro,  326,  457,  458 
Butschli,  95 


CALLUS,  413 

plant,  418 
Camera-eye,  160 
Cancer,  implantation  of,  437 
Canestrini  on  sex  determination, 

243 

Capillaries,  development  of,  133 
Carchesium,  45,  115 

specialization  in,  111 
Cardan,  22 
Carrel  on  grafting,  420,  430 

on  growth  of  tissue  in  vitro, 
326,  457,  458,  459 

on  kidney  transplantation,  425 
Cartilage,  regeneration  of,  414 
Castle  on  sex  determination,  246 
Caustics,  46 

Cecidomyia,  reproduction  in,  203 
Cell  association,  114 

centrosome  of,  101 

definition  of,  93 

division,  91,  102 
direct,  106 

granules  of,  95 

nucleus  of,  97 

wall,  99 
Cells,  composition  of,  95 

plant,  97 

reproductive,  183 


INDEX 


463 


Cells,  size  of,  94 
Cellulose,  90 

Central  nervous  system,  develop- 
ment of,  155,  165 
Centrosome,  101 
Cercomonas,  338 
Cestoda,  339 

Chambers  on  evolution,  287 
Chameleon,  effect  of  light  on,  58 
Chemical  stimulation,  46 
Chemicophysical   theory   of   he- 
redity, 274 

Chemicophysicists,  81 
Chemotropism,  46 
Chief  brain,  155 
Chiggerbuttons,  346 
Chlorophyl,  57 
Chloroplasts,  59,  97 
Cholera,    immunization    against, 

399 

Chromatin,  cell,  98 
Chromatophores,  97 
Chromoplasts,  59,  97 
Chromosomes,  98 

division  of,  105 

in  cell  division,  104 

reduction  of,  204 

separation  of,  105 
Cicatricial  tissue,  412 
Cilia,  76,  126 

oral,  77 

Ciliate  movements,  76 
Ciliates,  structure  of,  110 
Circulation,  .cytoplasmic,  72 
Circulatory  system,  development 

of,  129 

Cirripedia,  341 
Classes,  308 
Classification,  history  of,  301 

modern,  311 
Cleft  palate,  240 
Cleistogamous  flowers,  199 
Cnidocil,  128 
Cobra  venom,  375 
Coccidia,  339 

cell  wall  of,  100 

sporulation  in,  182 
Ccelenterata,  339 
Coelum,  223 

Cold.     See  Temperature. 
Cold-blooded  animals,   effect  of 

temperature  on,  39 


Coleoptera,  346 
Colon  bacillus,  334 
Colonial  aggregations,  115 
Commensalism,  328 
Complement,  380 
Compound  reflex  action,  169 
Conductivity,  67 
Conformity  to  type,  249 
Conjugation,  185 

in  spirogyra,  191,  192 
Connective   tissue,   regeneration 

of,  412 
Consciousness,  170,  174 

development  of,  174 

existance  of,  175 
Constant  characters,  302 
Contractile  movements,  80 

vacuoles,  73,  101 
Controller,  152,  153 
Coordination,  development  of, 

148 

Cornea,  160 

Corpuscles,  Meissner's,  157,  159 
Correns  on  sex  determination,  246 
Cosmical  relations  of  living  mat- 
ter, 17 

Creite  on  blood,  318 
Criteria  of  life,  31 
Crone,  88 
Cross-breeding  as  a  Lamarckian 

factor,  286 
Crustacea,  341 

Cultivation  of  tissue  in  vitro,  456 
Cuvier  on  evolution,  284 

on  paleontology,  304 
Cuvier's  classification,  306 
Cyclical  changes  of  protoplasm, 

32 

Cytisus,  grafting  in,  441 
Cytolysis  by  electricity,  63 
Cytomorphosis,  446 
Cytoplasm,  95 
Cytoplasmic  circulation,  72 
Cytotoxins,  317,  378 

DANIEL  on  grafting,  440 

Darwin  (Charles)  and  evolution, 
288 

Darwin  (Erasmus)  and  evolu- 
tion, 284 

Darwin's  theory  of  pangenesis, 
252 


464 


INDEX 


Daughter  star,  formation  of,  106 

Death,  443,  452,  456 

De  Barry,  186 

De  Graaf,  214 

De  Maillet,  284 

De  Vries  on  Mendel's  law,  271 
theory  of  mutation,  297 

Decadence,  443 

Definitive  host,  333 

Dendrites,  150 

Depressants,  37 

Determinants,  double,  265 
of  Weismann,  261 

Deutoplasm,  97 

Development,  science  of,  211 

Diatomacese,  fossil,  114 

Diatome  and  bacteria,   mutual- 
ism of,  53 

Dibothriocephalus  latus,  339 

Dicroccelium,  339 

Digestive    system,    development 
of,  123 

Dimorphism,  Weismann's  theory 
of,  265 

Dioncea,  36 

conductivity  in,  69 
mechanical  stimulation,  43 

Diptera,  343 

Dipylidium  canium,  339 

Discophora,  341 

Divergence,  282 

Dodders,  335,  336 

Dominant  characters,  270 

Doncaster  on  sex  determination, 
247 

Donors,  universal,  in  blood  trans- 
fusion, 324 

Drelincourt,  240 

Drosera,  conductivity  in,  69 
mechanical  stimulation,  43 

Dumas,  214 

Dusing  on  sex  determination,  243 

Dysentery,  parasite  of,  338 


EAR,  development  of,  161 

Earthworm,  127 
grafting  in,  422 
regeneration  in,  406 

Ectoderm,  219,  222 

Ectoparasites,  333 

Ectoplasm,  80 


Ectosarc,  100 

Effector,  152,  153 

Efferent  neuraxes,  152 

Egg,  structure  of,  216 

Eggs,  effect  of  temperature  on,  40 

Ehrlich  on  blood,  318 

Ehrlich's    lateral    chain    theory, 

380 
Electric  stimulation,  response  to, 

60 

Elements,  18 
Embryo,  211 
Embryology,  211 
Empedocles,  21,  282 
Endoparasites,  333 

transmission  of,  353 
Endosarc,  100 
Entamceba  histolytica,  338 
Enteron,  124 
Entoderm,  219,  222 
Entomophilous  plants,  200 
Eosinophile  cells,  95 
Epiblast,  222 
Epigenesis,  213,  214 
Epimorphosis,  406 
Epistylus,  115 
Epithelial     tissue,     regeneration 

of,  412 

Equatorial  plate,  105 
Ergot  of  rye,  334 
Erythrocytes,    development    of, 

135 
Eurotium    repens,    reproduction 

in,  186 
Eveling,  457 
Evolution,  283 
Evolutionary  sequence,  304 
Excretion,  organs  of,  146 
Excretory  system,   development 

of,  145 

Exteroceptors,  152 
Eye,  development  of,  159 


FALLOPIAN  tubes,  cilia  of,  79 

Families,  309 

Fasciola  hepaticum,  339 

Fernandez,  218 

Ferns,  reproduction  in,  198 

Fertilization  of  ovum,  207,  208 

processes  of,  217 
Filaria,  340,  357 


INDEX 


465 


Finsen's  light  and  tumors,  60 

Fischer,  433 

Fish  and  oxygen,  54 

Fishes,  heart  in,  137 

Fixateur,  379 

Flagella,  76,  126 

Flagellates,  movement,  76 

structure  of,  110 
Flame-cells,  147 
Fleas,  332,  345 
Flemming,  95,  102 
Flies,  parasitic,  343 
Floating  matter  of  Tyndall,  28 
Flowers,  mechanical  stimulation 

by  insects,  44 
Foetus,  211 
Food,  83 

stimulating  influence  of,  48 
Foot,  457 
Freckles,  60 
Freezing,  effects  of,  38 
Fungi  and  force  of  gravity,  64 


GALTON,  238,  254 

Galton's  theory  of  stirp,  255 

Galvanotropism,  60 

Gametes,  186,  206 

Gasteropoda,  341 

Gastrula,  222 

Gay,  323 

Gemmation,  181 

Gemmini,  206 

Gem  mules  of  Darwin,  252 

Generation,  spontaneous,  21 

Geology,  303 

Geotropism,  63 

Germ  plasm,  204 

of  Nageli,  258 

of  Weismann,  260 
Germinal  cells,  183 

differentiation   of,   from  so- 
matic cells,  119 
in  animals,  203 

disc,  224 
Gills,  138,  144 
Glossina,  244,  361 
Goethe,  284 
Gonads,  197 
Gonocytes.  197 
Graafian  follicles,  214 
Grafting,  420 

30 


Grafting  in  plants,  438 
Gravity,  effect  on  living 

63 

Growth,  83 
Guthrie  on  grafting,  426 


HABITUATION,  374 
Haeckel,  235 

Haffkine's  cholera  serum,  399 
Haller,  213 
Hammen,  212 
Hanes,  457,  458 
Haptophores,  381 
Hare-lip,  formation  of,  240 
Harrison  on  grafting,  435 

on  growth  of  tissue  in  vitro, 

326,  457,  458 
Hartsoeker,  214 
Harvest  bug,  351 
Harvey,  William,  211 
Harvey's  law,  28 
Hearing,  development  of,  161 
Heart,  batrachian  type  of,  141 

development  of,  137,  238 
Heat.     See  Temperature. 

stimulation,  response  to,  37 
Hektoen,  323 
Heliotropism,  54 
Helotism,  331 
Hemoglobin,  135 
Hemolysins,  317 
Hemolytic  serum,  379 
Hemophilia,  transmission  of,  265 
Henking  on  sex  determination, 

244 

Hensen  on  sex  determination,  243 
Hereditary  memory,  170 
Heredity,  169,  249 
Hermaphroditism,  189,  201 
Herpetomonas,  338 
Herrick,  164 

on  consciousness,  175 

windows  of,  157 
Berrick's  types  of  reflex  circuits, 

154 

Hertwig,  49,  53 
Heterochromosomes,  245 
Eleteromorphosis,  405 
Heterotype  mitosis,  206 
Heterozygous,  269 
Histology,  93 


466 


INDEX 


Holoblastic  egg,  216 

Holophyra,  338 

Holophytic  nutrition,  90 

Holothurians,    effect   of   gravity 
on,  67 

Holozoic  nutrition,  90 

Homologous  twins,  238 

Homomorphosis,  405 

Homonculus,  212 

Homotype  mitosis,  205,  206 

Homozygous,  269 

Hooker,  93 

Hook-worm,  340,  354 

Host,    condition   of   receptivity, 

367 

definitive,  333 
intermediate,  333 
of  parasite,  331 

Hume,  David,  on  evolution,  283 

Huxley  on  classification,  308 

Huxley's  criteria  of  life,  31 

Hyaloplasm,  95 

Hyami,  457 

Hybridization,        blood-relation- 
ship and,  325 

Hybrids,  Mendel's  study  of,  267 

Hydra,   digestive    apparatus   of, 

124 

grafting  in,  424 
mechanical  stimulation  of,  45 
nettling  cells  of,  126 
regeneration  in,  403 
reproduction  in,  194 
structure  of,  120 

Hydrophobia,     Pasteur's    treat- 
ment of,  396 

Hydrotropism,  50 

Hydrozoa,  339 

Hymenolepis  nana,  339 

Hymenoptera,  346 

Hyperchromatosis,  102 

Hypersensitivity,  377 

Hypoblast,  222 

Hypoderma,  344 

Hypopolar  field,  105 


ID,  261 

Idants,  261 

Idioplasm,  accessory,  263 

blastogenic,  263 

of  Nageli,  258 


Immune  body,  379 
Immunity,  364 

acquired,  373 

definition  of,  367 

general  considerations  of,  368 

natural,  370 

phenomena  of,  374 
Immunization,  376 
Importance  of  brain,  165-168 
Infection,  364 

cardinal  conditions  of,  366 
Infectious  disease,  immunity  to, 

389 

Infusoria,  parasitism  of,  338 
Ingebrigsten,  457,  459 
Innervation,  development  of,  148 
Insects,  effect  of  light  on,  59 

fertilization  of  plants  by,  200 
Instinctive  action,  170,  171 
Instincts,  168 

primary,  170 

secondary,  170 
Intelligence,  171 

lapsed,  170 

Intermediate  host,  333 
Interoceptors,  152 
Intestinal  worms,  dissemination 

of,  353 
In  vitro,   cultivation  of  tissues, 

456 

Irritability,  34 

Lsohemagglutinins    in    blood   re- 
lationship, 323 

Isohemolysins  in  blood  relation- 
ship, 323 
Itch,  351 

mite,  333 


JAGER  on  inheritance,  259 
James  on  instincts,  169,  170 
Jelly  fishes,  circulation  in,  131 
Jenner  and  vaccination,  392 
Jennings,  74 

on  instincts,  167,  169 
Joest  on  grafting,  422 
Jolly,  457 


KANT  and  Laplace,  nebular  hy- 
pothesis, 18 
Karstner,  323 


INDEX 


467 


Karyokinesis,  102 

Karyolysis,  99 

Karyomitome,  98 

Karyoplasm,  98 

Karyorrhexis,  99 

Katabolism,  83 

Kidney,  development  of,  147 
regeneration  of,  416 
transplanatation  of,  425,  430, 
431 

Kircher,  22 

Knauer  and  Grigorieff  on  trans- 
plantation, 429 

Knight,  64 

Kocher  on  transplantation,  428 

Korschinsky,  296 

Kiihne,  63 

Kupelweiser,  210 


LAMARCK  and  paleontology  304 

on  evolution,  284 

on  invertebrata,  307 
Lamarckian  factors,  285 
Lambert   on   cultivation   of   tis- 
sues in  vitro,  457,  458,  459 
Landois   on   blood   relationship, 

318 

Landsteiner,  323 
Lapsed  intelligence,  170 
Larva,  211 
Lateral  chain  theory  of  Adami, 

274 

of  Ehrlich,  380 
Lecithocele,  227 
Lee,  323 
Leeches,  341 
Leeuwenhoek,  23,  212 
Leibnitz,  283 
Lens,  crystalline,  160 
Leptus  autumnalis,  351 
Lessona  on  regeneration,  408 
Leucocytes,  electric  currents  and, 

61 

Leucoplasts,  59 

Lewes,  G.  H.,  on  instincts,  170 
Lewis  on  cultivation  of  tissues  in 

vitro,  457,  459 
Lexer,  430 
Lice,  332,  342 
Life,  criteria  of,  31 

manifestations  of,  34 


Life,  origin  of,  20 
Light,  stimulation  of,  54 
Linin,  98 

Linnaeus    and    binomial  nomen- 
clature, 310 

classification  of,  302 
Liver  flukes,  339 

regeneration  of,  416 
Living    matter,     cosmical    rela- 
tions, 17 

Lizards,  regeneration  in,  408 
Lock-jaw,  bacillus  of,  333 
Lock  on  Mendel's  law,  271 
Locomotion,  71 

appendages  of,  128 
Loeb,  63,  150,  169,  172 

on    cultivation    of    tissues    in 
vitro,  457 

on  geotropism,  66 

on  reproduction,  210 

on  skin  transplanatation,  433 
Longevity,  444 
Lungs,  development  of,  145 

regeneration  of,  416 


MACCALLUM  on  malarial  parasite, 

358 

Macronucleus,  98 
Macrophages,  388 
Magosphsera  planula,  116 
Malarial  parasite,  358,  359 
Mai  de  caderas,  344 
Malformations,  239 
Mallophaga,  342 
Malthus  on  evolution,  287 
Man,  effect  of  light  on  skin  of,  59 

of  temperature  on,  40 
Mange,  351 

Manifestations  of  life,  34 
Mann,  Gustav,  84 
Manson  (Sir  Patrick)  on  filaria, 
357 

on  malarial  parasite,  358 
Manubrium  of  Obelia,  131 
Mastigophora,  parasitism  of,  338 
Maturation,  206 
Maupas,  184 

on  sex  determination,  242 
Maximum  temperature,  39 
McCallum  on  grafting,  441 
McClung,  245 


INDEX 


Mechanical  stimulation,  response 

to,  43 

Medusae  of  Obelia,  196 
Megastoma  entericum,  338 
Meissner's  corpuscles,  157,  159 
Memory,  associative,  172 

hereditary,  170 
Mendel,  Gregor  Johann,  267 

on  sex  determination,  246 
Mendelian  proportions,  diagram 

explaining,  273 
Mendel's  law,  270 
Menopause,  449 
Meroblastic  egg,  216 
Mesenchyme,  121 
Mesoblast,  222 
Mesoderm,  226 
Metabolic  activity,  anabolic,  83 

katabolic,  83 
Metabolism,  81 
Metagenesis,  197 
Metaphase,  105 
Metaphyta,  93 
Metastasis,  90 
Metazoa,  93 
Metschnikoff  on  old  age,  446 

on  phagocytosis,  385 
Micellae,  258 
Microcytase,  388 
Microgromia  socialis,  115 
Micronucleus,  98 
Micro-organisms.     See  Bacteria. 
Microparasites.     See  Bacteria. 
Microphages,  388 
Mimosa,  36 

conductivity  in,  69 

effect  of  light  on,  56 

mechanical  stimulation,  43 

thermal  irritability  of,  42 
Mind,  176 

Minimum  temperature,  39 
Minot,  236 

on  senescence,  446 
Mistletoe,  337 
Mites,  350 
Mitochondria,  95 
Mitosch  on  grafting,  441 
Mitosis,  102 

heterotype,  206 

homotype,  205,  206 
Moisture,  influence  of,  50 
Molecular  death,  452,  456 


Monogenetic  reproduction,  181 
Monsters,  formation  of,  239 
Montague,  Lady  Mary  Wortley, 

391 

Morgan  (Lloyd)  on  instinct,  170 
Morgan  (T.  H.)  on  instincts,  169 

on  regeneration,  405,  406 
Morphalaxis,  406 
Morris  on  grafting,  429 
Morula,  221 
Mosquitoes,  345 

parasitism  of,  332 
Moss,  323 
Mother  star,  105 
Motion,  71 
Motor  neuron,  diagram  of,  151 

system,  development  of,  125 
Moulds  and  moisture,  51 

effect  of  gravity  on,  64 
Movement,  amoeboid,  74 

ciliate,  76 

contractile,  80 

cytoplasmic,  72 

flagellate,  76 
Mucor  mucedo,  reproduction  in, 

192,  193 

Murphy  (J.  B.),  438,  457 
Muscle  cells,  80,  129 
Muscular  tissue,  atrophic  changes 

in,  448 

regeneration  of,  414 
Mutation  theory  of  De  Vries,  297 
Mutilation  and  regeneration,  401 
Mutualism,  329 
Myeloplaxes,  99 
Myiasis,  343 
Myograph,  81 


NAGELI  on  heredity,  258 

Natural  immunity,  370 
selection,  290 

Nebular  hypothesis,  18 

Necator  americana,  340,  354 

Needham,  24 

Nematoda,  340 

Neo-Lamarckism,  295 

Nephridia,  147 

Nerve-fibers,  transmission  of  elec- 
tric currents,  63 

Nervous  system,  atrophic  changes 
in,  450 


INDEX 


Nervous  system,  development  of, 

148,  149,  155 
tissue,  regeneration  of,  414 

Nettling  cells,  126,  128 

Neuman,  218 

Neuraxis,  150 

Neuron,  150 

Neurone,  motor,  diagram  of,  151 

Neurophages,  451 

Neutrophilic   granules   of   leuco- 
cytes, 95 

Nose,  restoration  by  grafting,  427 

Nuclear  membrane,  98 
spindle,  105 

Nuclein,  98 

Nucleolus  of  the  cell,  98 

Nucleus  of  the  cell,  97 
division  of,  102 

Nussbaum,  259 

Nutrition,  holophytic,  90 
holozoic,  90 

Nuttall  and  Buchner,  387 
on  blood  relationship,  317 


OBELIA,  reproduction  in,  196 

Obligatory  parasite,  332 

Occasional  parasites,  332 

Oken's  classification,  307 

Old  age,  443 

Olfactory    organs,    development 

of,  163 

Omne  vivum  ex  vivo,  28 
Ontogenesis,  209 
Oogenesis,  204,  206 
Oogonium,  190 
Opalina,  338 
Opspnins,  389 
Optimum  temperature,  39 
Orbitolites,  113 
Orders,  309 
Organ  of  Corti,  163 
Organs  of  special  sense,  156,  157 
Origin  of  life,  20 
"Origin  of  Species,"  288 
Ornithodorus  moubata,  349 
Otocysts,  162 
Otoliths,  162 
Ottenburg,  323 
Ova,  204 
Ovid  on  spontaneous  generation, 

21 


Ovists,  212 

Ovum,  fertilization  of,  207,  208 

segmentation  of,  217 

mammalian,  100 
Owen,  202 
Ox-warble,  344 
Oxygen,  fish  and,  54 

stimulating  effects  of,  52 
Oxygenation  of  blood,  144 
Oxytropism,  52 
Oxyuris  vermicularis,  340,  354 


P.EDOGENESIS,  203,  211 

Paleontology,  303 

Pangenesis  of  Darwin,  252 

Paragonimus  westermanii,  339 

Paralinin,  98 

Paramoecia,   electric   stimulation 

of,  61 
Paramcecium,  conjugation  in,  183 

cytoplasmic  circulation  in,  73 

locomotion  in,  77 
Paranucleus,  98 
Paraplasm,  97 
Parasite,  331 

Parasites,  classification  of,  338 
Parasitism,  327 

proper,  331 
Parmelee,  167,  168,  169,  170,  172 

on  consciousness,  175 
Parthenogenesis,  189,  202 
Parthenogenetic       development, 

189,  202,  203 
Pasteur    (Louis)    and    infectious 


,393 
and  rabies,  396 

Pasteur's  experiments  on  spon- 
taneous generation,  26 

Patterson,  218 

Pediculoides  ventricosus,  350 

Perithecium,  186 

Pfeffer,  43,  47 

Phagocytosis,  385 

Phagolysis,  388 

Photic  stimulation,  54 

Photosynthesis,  89,  90 

Phyla,  308 

Phylogenetic  cells,  259 

Physicochemical    basis    of    vital 
phenomena,  321 

Physodes,  97 


470 


INDEX 


Phytoprecipitins,  317 
Pin-worm,  354 

Plague  serum  of  Haffkine,  399 
Planaria.  regeneration  in,  404 
Plant  cells,  97 

parasites,  346 

parasitism,  334 
Plantade,  212 
Plants  and  electric  currents,  60 

cell  wall  of,  100 

classification  of,  311 

conductivity  in,  69 

effect  of  light  on,  56 

fertilization  by  insects,  200 

geotropic  reactions  of,  64 

grafting  in,  438 

regeneration  in,  417 

reproduction  in,  199 
Plasmodia,  98,  112 
Plasmodium  malariir,  339,  358 
Plenciz,  24 

Plett  and  vaccination,  392 
Poisons,  46 

reactions  to,  374 

tolerance  of,  372 
Polar  bodies,  208 

field,  105 
Polioplasm,  95 

Polydactylism,  a  Mendelian  char- 
acteristic, 273 
Polyembryony,  217,  218 

accidental,  218 
Precipitation,  317,  318 

specific,  neasurement  of,  319 
Precipitins     in     blood     relation- 
ship, 318 

Preformat  ion  theory.  212 
Prevost,  214 
Primitive  groove,  224 

streak,  224 
Proglottides,  363 
Pronucleus,  208 
Prophase,  102 
Proprioceptors,  152 
Proskauer,  87 
Protandrism,  195 
Protandry,  199 
Protein,  84 
Prothallium,  198 
Protogyny,  200 
Protophyta,  93 
Protoplasm,  19,  31,  84 


Protoplasm,   electric  stimulation 
of,  61 

elements  of,  87 

of  the  cell,  95 

thermal  irritability  of,  40 
Protozoa,  93 
Pseudopodia,  75 
Pseudopods,  126 
Psychology,  179 
Pyrenin,  98 


QUARTER  evil,  vaccine  for,  396 


RABIES,    Pasteur   treatment   of, 

396 

Radiolaria,  structure  of,  112 
Rauber  on  fertilization,  259 
RauUn,  87 
Reason,  176 
Receptor,  152,  153 
Receptors,  381 

types  of,  152 
Recessive  characters,  270 
Recipients,     universal,  in  blood 

transfusion,  324 
Redi,  22 
Reduction  division,  206.  207 

of  chromosomes,  204 
Reflex,  153 

action,  46,  150,  153,  170 
compound,  169 
definition  of,  167 
elements  of,  152 
mechanism  of,  173,  176 

arc,  152,  153 

circuit,  152,  153 
types  of,  154 

nervous  action,  mechanism  of, 

173 
Regeneration,  401 

influences  governing,  410 

in  plants,  417 

of  blood-vessels,  412 

of  bone,  413 

of  epithelial  tissue,  412 

of  muscular  tissue,  414 

of  nervous  tissue,  414 

of  viscera,  415 
Reichert   and   Brown   on   blood 

relationship,  321 


INDEX 


471 


Relapsing  fever,  361 
Repair.     See  Regeneration. 
Reproduction,  83,  91,  181 

in  animals,  200 

in  Cecidomyia,  203 

in  ferns,  198 

in  plants,  199 

in  sponges,  197 
Reproductive  cells,  183 

system,  development  of,  123 
Respiratory     system,     develop- 
ment of,  143 
Rhipicephalus  boyis,  362 
Rhizopoda,  parasitism  of,  338 
Ribbert  on  transplantation,  428, 

432 

Rigidity,  36 
Rigor  mortis,  453 
Romanes,  235 

on  instinct,  170 
Ross   (Sir  Ronald)   on   malarial 

parasites,  358 
Rotifer,  cilia  of,  78 

mechanical  stimulation  of,  45 
Rotifera,  341 
Round  worms,  340 
Rous,  457,  458 
Rusts,  335 
Ruth,  457,  458 


SADLER  on  sex  determination,  241 

Sarcocystis,  339 

Sarcopsylla  penetrans,  345 

Sarcoptes  scabiei,  333 

Scabies,  351 

Scar,  412 

Schenk  on  sex  determination,  241 

Schistosoma  hematobium,  354 

Schleicher,  102 

Schleiden,  93 

Schroeder,  25 

Schultze,  25 

Schwann,  25,  93,  214 

Scolex,  356 

Seat- worm,  354 

Segmentation  cavity,  227 

of  ovum,  217 
Self-consciousness,  175 
Senescence,  443 
Setae,  locomotor,  128 
Sex  determination,  240-248 


Sex  determination,  x-element  in, 

245 
Sexual  organs,  atrophic  changes 

in,  449 

Shattuck,  323 

Shattuck  and  Ballance  on  carcin- 
oma, 437 
Sheep-scab,  351 
Sheep-tick,  343 

Sherrington  on  reflexes,  167,  168 
Sherrington's  types  of  receptors, 

152 

Side-chain  theory  of  Ehrlich,  380 
Sight,  development  of,  159 
Sitotrppism,  48 
Situs  inversus  yiscerum,  240 
Size    of    organisms,    relation    to 

complexity,  113 
Skin,  atrophic  changes  in,  449 
Skm-grafting,  427,  429 
Small-pox  and  vaccination,  391 
Smell,  development  of,  163 
Smut  of  corn,  335 
Somatic  cells,  203 

differentiation  of,  from  ger- 
minal cells,  119 

death,  452,  456 

Spallanzani,  25,  214,  407,  408 
Special  sense,  organs  of,  156 
Specialization  of  cells,  110 
Species,  309 

Spencer  (Herbert)  and  evolution, 
288 

on  heredity,  250 

on  instincts,  169 
Spermatocytes,  secondary,  206 
Spermatogenesis,  204 
Spermatozoa,  204 
Spermatozoits,  359 
Spermatozoon,  locomotion  of,  80 
Spillman  on  Mendel's  law,  271 
Spina  bifida,  development  of,  239 
Spirem,  104 
Spirillosis,  350 
Spirochaeta  duttoni,  350 
Spirochaetes,  362 
Spirogyra,  115 

conjugation  in,  191,  192 
Spleen,  regeneration  of,  416 
Sponges,  reproduction  in,  197 

structure  of,  120 
Spongioplasm,  95 


472 


INDEX 


Spontaneous  generation,  21 

Sporosacs,  196 

Sporozoa,  parasitism  of,  339 

Sporulation  in  coccidium,  182 

St.  Augustine,  283 

Starch,  formation  of,  89 

Stentor,  nucleus  of,  99 

regeneration  in,  403 
Stigma,  199 
Stimulation,  chemical,  46 

electric,  60 

food,  48 

gravity,  63 

light,  54 

mechanical,  43 

oxygen,  52 

photic,  54 

thermal,  37 

water,  50 

Stimuli,  action  of,  34 
Stirp,  255 
Stomoxys,  344,  361 
Strasburger,  190 

on  grafting,  441 
Strepsiptera,  346 
Strobila,  356 

Structural  relationship,  301 
Sundew,  43.     See  Droaera. 
Sun's  rays,  effect  of,  54 
Surra,  344 
Suspensorium,  193 
Swammerdam,  214 
Symbiont,  328 
Symbiosis,  328 

blood-relationship  and,  326 
Sympathetic  system,  179 


TADPOLE,  grafting  in,  422,  435 

Tsenia  echinococcus,  340,  356 
saginata,  339,  355 
solium,  339 

Tanaka,  457 

Tape-worms,  339 
life  of,  355 

Taste,  development  of,  164 

Tactile'corpuscles,  157,  159 
sense,  development  of,  157 

Tchistowich  on  blood,  318 

Telophase,  106 

Temperature  and  the  regenera- 
tive function,  409 


Temperature,  effect  of,  on  bac- 
teria, 38 
on  eggs,  40 
on  man,  40 

maximum,  39 

minimum,  39 

optimum,  39 

response  to,  37 
Tentacles,  127 
Testis,  204 
Tetanus,  36 

bacillus,  333 

toxic  product  of,  366 
Texas  cattle  fever,  349,  362 
Thermal    stimulation,    response 

to,  37 

Thermotropism,  37 
Thigmotropism,  43 
Thury  on  sex  determination,  243 
Ticks,  347 

Tissue,  growth  of,  in  vitro,  326, 457 
Toadflax,  bastard,  337 
Tooth,  transplantation  of,  431 
Touch,  sense  of,  46 

development  of,  157 
Toxins,  47,  374 
Toxophores,  381 
Transfusion  of  blood,  323 

groups  for,  324 

Transplantation  in  surgery,  428 
Treat,  242 
Trematoda,  339 
Treviranus,  284 
Trichinella  spiralis,  340 
Trichocephalus  dispar,  340 
Trichodinae,  338 
Trichomonas,  338 
Triploblastic  larvse,  222 
Trombidium,  351 
Trophoplasm,  258 
Tropical  dysentery,  parasite  of, 

338 

Tropism,  37 
Trypanosomes,  338,  361 

flagella  of,  79 
Tsetse  fly,  361 

Tuberculosis,  bacillus  of,  334 
Tumors,  implantation  of,  437 
Twins,  23.9 

Tyndall,    experiments   on    spon- 
taneous generation,  26 
Type,  conformity  to,  249 


INDEX 


473 


UHLENHUTH  on  blood,  318 
Ullman  on  grafting,  425 
Ulothrix  zonata,  sporulation  in, 

189 

Units  of  Spencer,  251 
Universal  donors  in  blood  trans- 
fusion, 324 

recipients  in  blood  transfusion, 
324 

VACCINATION,  392 

Vaccine  treatment,  399 

Vacuoles,  contractile,  73,  101 
in  cells,  101 

Van  Dusch,  25 

Van  Helmont,  22 

Variety,  309 

Vascular    system,     development  | 

of,  129 
tubules  of  plants,  130 

Vaucheria,  reproduction  in,  189 

Vegetable  cells,  97 

Vena?  cavse,  141 

Venus'  fly-trap,  35,  36 

Vernon  on  sex  determination,  243 

Verworn,  153 

Vesication,  60 

Virgil    on    spontaneous    genera- 
tion, 21 

Virulence,  364 

Viscera,  regeneration  of,  415 

Vision,  development  of,  159 

Vitalists,  81 

Vitreous  body,  160 

Volvox,  120 
globator,  117,  119 

Von  Baer,  214,  234 
on  classification,  308 

Von    Eiselberg    on    transplanta- 
tion, 428 

Vorticella,  conductivity  in,  70 
mechanical  stimulation  of,  45 
nucleus  of,  99 

WAGNER,  203 
Waldeyer,  104 


Wallace  and  evolution,  288 
Warm-blooded  animals,  effect  of 

temperature  on,  39 
Wassermann    and    Schultze    on 

blood,  318 
Water,  stimulating  influence  of, 

50 

vascular  system,  131 
Weismann  on  chromosomes,  104 
on  Spencer's  theory  of  heredity, 

251 
Weismann's     theory     of     germ 

plasm,  259 

White  blood-corpuscles,  develop- 
ment of,  134 
granules  of,  95 
Wilson    on    sex    determination, 

244 

on  Weismann's  theory,  266 
Wilson  (H.  V.),  404 
Wilt  disease  of  cucumbers,  334 
Windows  of  Herrick,  157 
Wolff  (Caspar  Frederick),  213 
Woodruff,  184 
Worms,  locomotor  apparatus  of, 

128 

vascular  system  of,  131 
Wounds,  healing  of,  414 
Wright  and  opsonins,  389 


X-ELEMENT  in  sex  determination, 
245 


YERKES,  172 

Yolk,  216 

Young  on  sex  determination,  242 

Youth,  91 


ZONA  pellucida,  100 
Zooprecipitins,  317 
Zoospore,  190 
Zygospore,  193 
Zygotes,  189 


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